Apparatus and methods for detecting overlay errors using scatterometry

ABSTRACT

Disclosed is a method of determining an overlay error between two layers of a multiple layer sample. For each of a plurality of periodic targets target that each have a first structure formed from a first layer and a second structure formed from a second layer of the sample, a plurality of optical signals are measured at a plurality of incident angles, wherein there are predefined offsets between the first and second structures. An overlay error is then determined between the first and second structures by analyzing the measured optical signals at the plurality of incident angles from the periodic targets using a scatterometry overlay technique based on the predefined offsets without using a calibration operation.

CROSS REFERENCE TO RELATED PATENT APPLICATION

This application claims priority of the following co-pending U.S.Provisional Patent Applications: (1) Application No. 60/504,093,entitled APPARATUS AND METHODS FOR DETECTING OVERLAY ERRORS USINGSCATTEROMETRY, by Walter D. Mieher, filed 19 Sep. 2003, (2) ApplicationNo. 60/449,496, entitled METHOD AND SYSTEM FOR DETERMINING OVERLAYERRORS BASED ON SCATTEROMETRY SIGNALS ACQUIRED FROM MULTIPLE OVERLAYMEASUREMENT PATTERNS, by Walter D. Mieher, filed 22 Feb. 2003, and (3)Application No. 60/498,524, filed 27 Aug. 2003, entitled “METHOD ANDAPPARATUS COMBINING IMAGING AND SCATTEROMETRY FOR OVERLAY METROLOGY”, byMike Adel.

This application is also a continuation-in-part of U.S. application Ser.No. 10/729,838, by Walter D. Mieher et al. filed 5 Dec. 2003. Theseapplications are herein incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

The present invention relates to determination of overlay betweenstructures formed in single or multiple layers. More particularly, itrelates to determining overlay based on diffraction of radiationinteracting with such structures.

In various manufacturing and production environments, there is a need tocontrol alignment between various layers of samples, or withinparticular layers of such samples. For example, in the semiconductormanufacturing industry, electronic devices may be produced byfabricating a series of layers on a substrate, some or all of the layersincluding various structures. The relative position of such structuresboth within particular layers and with respect to structures in otherlayers is relevant or even critical to the performance of completedelectronic devices.

The relative position of structures within such a sample is sometimescalled overlay. Various technology and processes for measuring overlayhave been developed and employed with varying degrees of success. Morerecently, various efforts have been focused on utilizing radiationscatterometry as a basis for overlay metrology.

Certain existing approaches to determining overlay from scatterometrymeasurements concentrate on comparison of the measured spectra tocalculated theoretical spectra based on model shape profiles, overlay,and film stack, and material optical properties (n,k dispersion curves),or comparison to a reference signal from a calibration wafer.

Existing approaches have several associated disadvantages. For example,a relatively large number of parameters must be included in the profile,overlay, and film modeling to accurately determine the overlay. Forexample, in some approaches using simple trapezoidal models for both theupper and lower layer profiles, the minimum number of pattern parametersthat must be included is seven, including overlay. If film thicknessesvariation is included in the model, the number of parameters increasescorrespondingly. A large number of parameters could require increasedprocessing resources, may introduce corresponding errors, and may delaythe results, thereby possibly decreasing throughput and increasinginefficiencies and costs. For example, comparison of a measured spectrumto calculated reference spectra takes longer with more parameters,whether a library-based approach is used or a regression approach isused.

Another disadvantage of certain existing approaches to determination ofoverlay based on scatterometry is the detailed knowledge of the filmstack, film materials, and pattern element profiles that may be requiredto determine accurate theoretical spectra to compare to the measuredspectra.

Yet another disadvantage of certain existing approaches to determinationof overlay based on scatterometry is the accurate knowledge of thescatterometry optical system that may be required to determine accuratetheoretical spectra to compare to the measured spectra.

Therefore, in light of the deficiencies of existing approaches todetermination of overlay based on scatterometry, there is a need forimproved systems and methods for determination of overlay based onscatterometry.

SUMMARY OF THE INVENTION

Accordingly, mechanisms are provided for determining overlay errorbetween two layers of a sample using improved scatterometry overlaytechniques. In one embodiment, the scatterometry technique includeslinear based techniques for determining overlay which can be performedwithout utilization of calibration data (e.g., generated from models orfrom calibration test wafers). In another implementation, ascatterometry technique is accomplished using a phase based approach.The phase based approach may also be practiced without using calibrationdata. Additionally, several specific improvements for implementingscatterometry techniques (e.g., linear or phase based) are disclosed.These improved scatterometry technique implementations may be performedseparately or combined together in any manner. Uses of such overlayerror, as well as novel targets from which to determine overlay usingscatterometry overlay techniques, are also disclosed.

In one embodiment, a method for determining an overlay error between atleast two layers in a multiple layer sample is disclosed. An imagingoptical system is used to measure a plurality of measured opticalsignals from a plurality of periodic targets on the sample. The targetseach have a first structure in a first layer and a second structure in asecond layer, and there are predefined offsets between the first andsecond structures. The imaging optical system is configured to have anillumination and/or collection numerical aperture (NA) and/or spectralband selected so that only a 0^(th) diffraction order is collected andmeasured for the plurality of measured optical signals. A scatterometryoverlay technique is then used to analyze the measured optical signalsof the periodic targets and the predefined offsets of the first andsecond structures of the periodic targets to thereby determine anoverlay error between the first and second structures of the periodictargets.

In another method embodiment for determining an overlay error between atleast two layers in a multiple layer sample, a plurality of periodictargets that each have a first structure in a first layer and a secondstructure in a second layer, wherein there are predefined offsetsbetween the first and second structures, are provided. The methodincludes (a) using an optical system having a broadband source forgenerating an optical incident beam having multiple wavelengths, adetector for detecting a measured signal from the sample in response tothe incident beam and a filter for selectively passing particular one ormore wavelengths of the output signal to the detector, directing atleast one radiation beam towards each target to measure a plurality ofmeasured signals from the periodic targets while adjusting the filter soas to pass a particular one or more wavelengths of the measured signalsthrough the filter towards the detector in the form of a pluralityfiltered signals; and (b) using a scatterometry overlay technique toanalyze the filtered signals of the periodic targets and the predefinedoffsets of the first and second structures of the periodic targets tothereby determine an overlay error between the first and secondstructures of the periodic targets.

In another method embodiment, a plurality of optical signals aremeasured at a plurality of incident angles for each of a plurality ofperiodic targets target that each have a first structure formed from afirst layer and a second structure formed from a second layer of thesample. There are predefined offsets between the first and secondstructures. An overlay error is then determined between the first andsecond structures by analyzing the measured optical signals at theplurality of incident angles from the periodic targets using ascatterometry overlay technique based on the predefined offsets withoutusing a calibration operation.

In yet another method embodiment, a first optical signal is measuredusing a first ellipsometer or a first reflectometer and a second opticalsignal is measured using a second ellipsometer or a second reflectometerfor each of a plurality of periodic targets that each have a firststructure formed from a first layer and a second structure formed from asecond layer of the sample. In this embodiment, there are alsopredefined offsets between the first and second structures. The overlayerror between the first and second structures is then determined byanalyzing the measured first and second optical signals from theperiodic targets using a scatterometry overlay technique based on thepredefined offsets.

In another method approach, for a plurality of periodic targets thateach have a first structure formed from a first layer and a secondstructure formed from a second layer of the sample, an interferometer isimployed to modulate substantially a plurality of wavelengths of abroadband source. One or more images of the periodic targets are thenacquired. There are predefined offsets between the first and secondstructures. An overlay error is then determined between the first andsecond structures by analyzing the one or more acquired images from theperiodic targets using a scatterometry overlay technique based on thepredefined offsets.

In another scatterometry overly technique, an optical system having atunable laser is employed at a plurality of settings for a plurality ofperiodic targets that each have a first structure formed from a firstlayer and a second structure formed from a second layer of the sample. Aplurality of optical signals are then measured at the plurality oftunable laser settings for each of the periodic targets. There arepredefined offsets between the first and second structures. An overlayerror is then determined between the first and second structures byanalyzing the measured optical signals from the periodic targets using ascatterometry overlay technique based on the predefined offsets.

In another scatterometry overlay embodiment, an optical system having aspatial filter for selectively filtering an optical signal measured isemployed to thereby measure an optical signal from each of a pluralityof periodic targets that each have a first structure formed from a firstlayer and a second structure formed from a second layer of a samplewhile spatially filtering at least a portion of at least one of themeasured optical signals. There are predefined offsets between the firstand second structures. An overlay error is then determined between thefirst and second structures by analyzing the measured optical signalsfrom the periodic targets using a scatterometry overlay technique basedon the predefined offsets.

In another scatterometry overlay method, a plurality of periodic targetsthat each have a first structure in a first layer and a second structurein a second layer are provided. There are predefined offsets between thefirst and second structures. An ellipsometer having a polarizationmodulator is used to measure a plurality of measured signals from theperiodic targets. A scatterometry overlay technique is then used toanalyze the measured signals of the periodic targets and the predefinedoffsets of the first and second structures of the periodic targets tothereby determine an overlay error between the first and secondstructures of the periodic targets.

In another embodiment, a plurality of theoretical scatterometry signalsare generated for a plurality of target configurations and/or processconditions and/or overlay errors configurations using a model orcalibrated data. The plurality of theoretical scatterometry signals andtheir associated target configurations and/or process conditions and/oroverlay errors are stored. For each of a plurality of measured periodictargets that each have a first structure formed from a first layer and asecond structure formed from a second layer of the sample, an opticalsignal is measured. There are predefined offsets between the first andsecond structures. A measured overlay error is measured between thefirst and second structures by analyzing the measured optical signals atthe plurality of incident angles from the periodic targets using ascatterometry overlay technique based on the predefined offsets withoutusing a calibration operation. The measured scatterometry signal iscompared with the stored measured scatterometry signals to obtain acharacteristic of the measured periodic targets or process condition forsuch measured periodic targets based on a substantially matchingtheoretical overlay value.

In one scatterometry overlay embodiment, any combination of thefollowing instruments may be used to measure optical signals for aplurality of periodic targets that each have a first structure formedfrom a first layer and a second structure formed from a second layer ofthe sample, wherein there are predefined offsets between the first andsecond structures:

an imaging reflectometer, an imaging spectroscopic reflectometer, apolarized spectroscopic imaging reflectometer, a scanning reflectometersystem, a system with two or more reflectometers capable of paralleldata acquisition, a system with two or more spectroscopic reflectometerscapable of parallel data acquisition, a system with two or morepolarized spectroscopic reflectometers capable of parallel dataacquisition, a system with two or more polarized spectroscopicreflectometers capable of serial data acquisition without moving thewafer stage or moving any optical elements or the reflectometer stage,imaging spectrometers, imaging system with wavelength filter, imagingsystem with long-pass wavelength filter, imaging system with short-passwavelength filter, imaging system without wavelength filter,interferometric imaging system, imaging ellipsometer, imagingspectroscopic ellipsometer, a scanning ellipsometer system, a systemwith two or more ellipsometers capable of parallel data acquisition, asystem with two or more ellipsometers capable of serial data acquisitionwithout moving the wafer stage or moving any optical elements or theellipsometer stage, a Michelson interferometer, and a Mach-Zehnderinterferometer, a Sagnac interferometer, a scanning angle of incidencesystem, a scanning azimuth angle system. An overlay error is thendetermined between the first and second structures by analyzing themeasured optical signals from the periodic targets using a scatterometryoverlay technique based on the predefined offsets.

In another embodiment, a spectroscopic ellipsometer is employed tothereby measure an optical signal from each of a plurality of periodictargets that each have a first structure formed from a first layer and asecond structure formed from a second layer of the sample. There arepredefined offsets between the first and second structures. An overlayerror between the first and second structures is then determined byanalyzing the measured optical signals from the periodic targets using ascatterometry overlay technique based on the predefined offset. Inanother aspect, a laser ellipsometer having a photoelastic modulator isemployed instead to measure the optical signals.

In yet another implementation, an optical system is used to measure aplurality of measured optical signals from a plurality of periodictargets on the sample. The periodic targets each have a first structurein a first layer and a second structure in a second layer, and there arepredefined offsets between the first and second structures. The periodictargets are arranged in same x or y direction. A scatterometry overlaytechnique is then used to analyze the measured optical signals of theperiodic targets and the predefined offsets of the first and secondstructures of the periodic targets to thereby determine an overlay errorbetween the first and second structures of the periodic targets.

In another implementation, the method for determining overlay includes(a) using an optical system to measure a plurality of measured opticalsignals from a plurality of periodic targets on the sample, wherein theperiodic targets each have a first structure in a first layer and asecond structure in a second layer, wherein there are predefined offsetsbetween the first and second structures, wherein the optical signals aremeasured at a same focus setting of the optical system withoutrefocusing; and (b) using a scatterometry overlay technique to analyzethe measured optical signals of the periodic targets and the predefinedoffsets of the first and second structures of the periodic targets tothereby determine an overlay error between the first and secondstructures of the periodic targets. In another embodiment, the measuredoptical signals are each in the form of a line image and not necessarilya same focus setting.

In another implementation of a scatterometry overlay technique, anoptical system is used to measure a plurality of measured opticalsignals from a plurality of periodic targets that each have a firststructure formed from a first layer and a second structure formed from asecond layer on the sample. Each first and second structure of eachtarget are designed to have a predefined offset with respect to eachother. A scatterometry overlay technique is used to analyze the measuredoptical signals of the periodic targets and the predefined offsets ofthe first and second structures of the periodic targets to therebydetermine a first overlay error between the first and second structuresof the periodic targets. A model based technique is also used to analyzethe measured optical signals of the periodic targets to therebydetermine a second overlay error between the first and second structuresof the periodic targets.

In another implementation, a sample having a plurality of periodictargets that each have a first structure in a first layer and a secondstructure in a second layer is provided. There are predefined offsetsbetween the first and second structures. Using scatterometry overlaymetrology, scatterometry overlay data is obtained from a first set ofthe periodic targets based on one or more measured optical signals fromthe first target set on the sample. Using an imaging overlay metrology,imaging overlay data is obtained from a second set of the periodictargets based on one or more image(s) from the second target set on thesample.

In another embodiment, the invention pertains to a method for aligningan imprint lithography mask with a semiconductor wafer. The methodincludes (a) aligning a plurality of periodic alignment marks of themask with a plurality of alignment marks on the wafer; (b) using anoptical system to measure a plurality of measured optical signals from aplurality of the periodic alignment marks on the mask and on the wafer,wherein the periodic alignment marks each have a first structure in afirst layer and a second structure in a second layer, wherein there arepredefined offsets between the first and second structures; (c) using ascatterometry overlay technique to analyze the measured optical signalsof the periodic alignment marks and the predefined offsets of the firstand second structures of the periodic alignment marks to therebydetermine an overlay error or mask registration error between the maskand the wafer; and (d) repeating operations (a) through (c) until theoverlay error or mask registration error equals a predetermined offsetor is within a predetermined range of mask misregistration values.

In another implementation, a combined scatterometry mark is disclosed.The mark includes a scatterometry critical dimension (CD) or profiletarget capable of being measured to determine CD or profile informationand a scatterometry overlay target disposed over the scatterometry CD orprofile target, the scatterometry overlay target cooperating with thescatterometry CD or profile target to form a scatterometry mark capableof being measured to determine overlay. In another aspect, the inventionpertains to a single metrology tool that includes a scatterometryoverlay measurement system configured to measure overlay; and a CD-SEMsystem configured to measure critical dimension. In another embodiment,the invention pertains to a method of producing a combined criticaldimension and overlay mark. A CD target is formed in a first layer of asample, and an overlay target is formed in a second layer of the sample,the overlay target being formed over the CD target. In yet anotherembodiment, the invention pertains to a method for determining CD in onelayer and an overlay error between at least two layers in a multiplelayer sample. The method includes performing scatterometry measurementson a CD target in order to determine CD, the CD target being formed in afirst layer of the sample, and performing scatterometry measurements onan overlay mark in order to determine overlay error, the overlay markcomprising an overlay target formed in a second layer of the sample andthe CD target formed in the first layer of the sample, the overlaytarget being disposed over the CD target.

In yet another implementation, a scatterometry mark configured fordetermining overlay error is disclosed. The mark includes (a) aplurality of periodic targets that each have structures on a first andsecond layer, wherein there are predefined offsets between the first andsecond structures so that overlay error between the first and secondstructures may be determined by analyzing a plurality of measuredoptical signals from each target; and (b) a plurality of targets thateach have third structures on a third layer that is underneath the firstand second layer, wherein the third structures are perpendicular to thefirst and second-structures. In an alternative embodiment, a targetstructure for measuring overlay between a second periodic structure anda third periodic structure are both disposed above a first line gratingwhere the first grating is oriented in a first direction and the secondand third periodic structure are oriented in a second direction, thesecond direction being substantially orthogonal to the first direction.In another aspect, a method of determining overlay error includesmeasuring the target structure described above with a scatterometryoverlay technique to determine overlay error between the second andthird periodic structures.

These and other features and advantages of the present invention will bepresented in more detail in the following specification of the inventionand the accompanying figures which illustrate by way of example theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the relative distribution of designed overlay offsetsXa, Xb, Xc, and Xd for corresponding interlayer patterns (overlaytargets) A, B, C, and D according to an embodiment of the presentinvention.

FIG. 2( a) is a side view illustration of a patterned top layer L2 beingoffset by an amount +F from a patterned bottom layer L1 in accordancewith one embodiment of the present invention.

FIG. 2( b) is a side view illustration of a patterned top layer L2 beingoffset by an amount −F from a patterned bottom layer L1 in accordancewith one embodiment of the present invention.

FIG. 2( c) is a side view illustration of a patterned top layer L2 beingoffset by an amount +F+f0 from a patterned bottom layer L1 in accordancewith one embodiment of the present invention.

FIG. 2( d) is a side view illustration of a patterned top layer L2 beingoffset by an amount −F+f0 from a patterned bottom layer L1 in accordancewith one embodiment of the present invention.

FIG. 2( e) is a side view illustration of a patterned top layer L2 beingoffset by an amount +F+f0+E from a patterned bottom layer L1 inaccordance with one embodiment of the present invention.

FIG. 2( f) is a side view illustration of a patterned top layer L2 beingoffset by an amount −F+f0+E from a patterned bottom layer L1 inaccordance with one embodiment of the present invention.

FIG. 3( a) is a flow diagram illustrating a procedure for determiningoverlay in accordance with one embodiment of the present invention.

FIG. 3( b) shows a graphical representation of an approach todetermination of overlay according to an embodiment of the presentinvention.

FIG. 4 is a diagrammatic representation of a conventional microscopicimaging system.

FIG. 5( a) is diagrammatic representation of a microscopic imagingsystem comprising a wavelength selection device, illuminationpolarization control, and polarization analyzer in accordance with afirst embodiment of the present invention.

FIG. 5( b) is diagrammatic representation of a microscopic imagingsystem comprising a wavelength modulation device, illuminationpolarization control, and polarization analyzer in accordance with asecond embodiment of the present invention.

FIG. 5( c) is diagrammatic representation of a microscopic imagingcomprising a illumination polarization control, polarization analyzer,and wavelength selection device, in accordance with a third embodimentof the present invention.

FIG. 5( d) is diagrammatic representation of a microscopic imagingsystem comprising a illumination polarization control, polarizationanalyzer, and wavelength modulation device in accordance with a fourthembodiment of the present invention.

FIG. 5( e) is a top view representation of an imaging spectrometer,multiple site field-of-view example in accordance with one embodiment ofthe present invention.

FIG. 5( f) is a diagrammatic representation of a fixed, discrete channeloptical system in accordance with a fifth embodiment of the presentinvention.

FIG. 5( g) is a diagrammatic representation of the aperture mirror ofFIG. 5( f) in accordance with one embodiment of the present invention.

FIG. 5( h) is a top view representation of an imaging spectrometer,multiple site field of view example with aperture components sent tospectrometers in accordance with one embodiment of the presentinvention.

FIG. 6 is a diagrammatic representation of a system for selecting one ormore wavelength ranges in accordance with one embodiment of the presentinvention.

FIG. 7 is a diagrammatic representation of a simultaneous, multipleangle of incidence ellipsometer.

FIG. 8 is a schematic view of a spectroscopic scatterometer system inaccordance with one embodiment of the present invention.

FIG. 9( a) shows a plurality of targets placed substantially-collinearlyalong either an X-direction or a Y-direction, wherein in this examplehalf of the targets are placed so as to measure overlay in the xdirection and half of the targets are placed so as to measure overlay inthe y direction, in accordance with a first embodiment of the presentinvention.

FIG. 9( b) shows four targets disposed collinearly along theX-dimension, and four targets disposed collinearly along the Y-dimensionin accordance with a second embodiment of the present invention.

FIG. 10 is a diagrammatic top view representation of a system forobtaining a line image of a plurality of targets in accordance with oneembodiment of the present invention.

FIG. 11 a is a top view representation of a first combination imagingand scatterometry target embodiment.

FIG. 11 b is a top view representation of a second combination imagingand scatterometry target embodiment.

FIG. 11 c is a top view representation of a third combination imagingand scatterometry target embodiment.

FIG. 11 d illustrates a combinational imaging and scatterometry systemin accordance with a first embodiment of the present invention.

FIG. 11 e illustrates a combinational imaging and scatterometry systemin accordance with a second embodiment of the present invention.

FIG. 11 f illustrates a combinational imaging and scatterometry systemin accordance with a third embodiment of the present invention.

FIG. 12 is a diagram of a combined mark, in accordance with oneembodiment of the present invention.

FIGS. 13A-13D show variations of a combined metrology tool, inaccordance with several embodiments of the present invention.

FIG. 14 is a flow diagram using a combined metrology tool, in accordancewith one embodiment of the present invention.

FIG. 15 is a perspective diagrammatic view of overlay line targets withL1 and L2 line elements perpendicular to underlying line grating L0 inaccordance with one embodiment of the present invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Reference will now be made in detail to a specific embodiment of theinvention. An example of this embodiment is illustrated in theaccompanying drawings. While the invention will be described inconjunction with this specific embodiment, it will be understood that itis not intended to limit the invention to one embodiment. On thecontrary, it is intended to cover alternatives, modifications, andequivalents as may be included within the spirit and scope of theinvention as defined by the appended claims. In the followingdescription, numerous specific details are set forth in order to providea thorough understanding of the present invention. The present inventionmay be practiced without some or all of these specific details. In otherinstances, well known process operations have not been described indetail in order not to unnecessarily obscure the present invention.

In general, the present invention provides techniques for determiningoverlay based on scatterometry measurements of any number ofscatterometry overlay (SCOL) targets. One implementation of ascatterometry technique which is referred to herein as the “linearapproach” will first be described. Alternative scatterometry approaches,such as a “phase approach” are also described. Finally, a number ofscatterometry based improvements will then be described. Although theseimprovements are mainly described in relation to the linearscatterometry approach, these scatterometry improvements may beimplemented using any other suitable scatterometry technique or approachsuch as the phase approach described herein. Additionally, although onlycertain specific combinations of improvements are described herein asbeing implemented together, any number of the improvements describedherein may be combined and implemented together.

An aspect of the present invention provides a set of four scatterometryoverlay targets (although in other embodiments more or less than fourmay be used) which have been formed on a sample or workpiece, such as asemiconductor device. A pattern could also be described as a “pattern orinterlayer pattern”, with the two terms being synonymous under mostcircumstances. In a particular implementation, the sample has two ormore layers of a semiconductor device, and the targets are utilized toprovide a measure of the placement accuracy of various structurescomprised in the device. Commonly, placement accuracy is characterizedby measurement of an overlay error between two different layers of thesemiconductor device. More generally, overlay error can be measuredbetween two different patterns generated by different pattern exposuresteps.

In a specific embodiment, a set of four targets are provided, and eachtarget includes two sets of structures on two different layers which areoffset from each other. In a specific implementation, an offset may bedefined as the sum or the difference of two separate distances: a firstdistance F and a second distance f0, with F being greater than f0.Denoting the four targets as “target A”, “target B”, “target C” and“target D”, the corresponding predetermined offsets for each of thesetargets may be defined as follows for a specific target design:

-   -   Xa=+F+f0 (for target A),    -   Xb=−F+f0 (for target B),    -   Xc=+F−f0 (for target C), and    -   Xd=−F−f0 (for target D).        The offsets for Xa through Xd may be any suitable value for        practicing the techniques of the present invention so as to        determine overlay. For example, Xa and Xb may have different        values of f0 than Xc and Xd.

FIG. 1 illustrates the distribution of offsets Xa, Xb, Xc and Xd alongthe x axis in a particular implementation of the invention. As shown,offsets Xa and Xc are both positive with Xa being larger than Xc. Incontrast, offsets Xb and Xd are both negative with Xd being morenegative than Xb. The offsets may be defined from a position in the unitcell of the first structure. If a symmetry position exists in the unitcell, it may be preferable to define the offsets from the symmetryposition. Alternatively, the offsets may be defined from a position inthe unit cell of the second structure but care should be taken to agreewith the convention of overlay measurements being defined as theposition of the L2 pattern (or second exposed pattern) measured withrespect to the position of the L1 pattern (or second exposed pattern).

The number of targets and the magnitude and sense of their correspondingoffsets may be chosen in any suitable manner so that the techniques ofthe present invention may be practiced to determine overlay error. Aspecific set of targets and their corresponding offsets are describedbelow in relation to FIGS. 2( a) through 2(f). It should be readilyapparent that there are numerous combinations of targets and offsetvalues which may be utilized to practice the techniques and utilize thesystems of the present invention.

FIG. 2( a) is a side view illustration of a patterned top layer L2 beingoffset by an amount F from a patterned bottom layer L1 in accordancewith one embodiment of the present invention. Each layer L1 and L2 ispatterned into a set of structures. A structure may include any suitablefeature, such as a line, trench or a contact. A structure may bedesigned to be similar to a semiconductor device feature. A structuremay also be formed from a combination of different features. Further, astructure may be located on any layer of the sample, e.g., either abovethe top layer of the sample, within any layer of the sample, orpartially or completely within a layer of the sample. In the illustratedembodiment of FIG. 2( a), layer L1 includes the complete structures 204a-c, while layer L2 includes the complete structures 202 a-c.Construction of scatterometry overlay targets structures and methods forproducing them are described in U.S. patent application, havingapplication Ser. No. 09/833,084, filed 10 Apr. 2001, entitled “PERIODICPATTERNS AND TECHNIQUE TO CONTROL MISALIGNMENT”, by Abdulhalim, et al.,which application is herein incorporated by reference in its entirety.

As shown, the structures of the top layer L2 are offset by an amount Ffrom the structures of the bottom layer L1. The structures of the twooffset layers may be located within adjacent layers or have any suitablenumber and types of layers disposed in between the two offset layers.FIG. 2( a) also shows three films T1, T2, and T3 between patternedlayers L1 and L2 and their corresponding structures. To the extent thatany other layers exist between the two layers having the structures,these other layers exhibit at least a minimum degree of transmission forelectromagnetic radiation to permit propagation of the radiation betweenthe layers having the structures.

FIG. 2( b) is a side view illustration of a patterned top layer L2 beingoffset by an amount −F from a patterned bottom layer L1 in accordancewith one embodiment of the present invention. FIG. 2( c) is a side viewillustration of a patterned top layer L2 being offset by an amount +F+f0from a patterned bottom layer L1 in accordance with one embodiment ofthe present invention. In one embodiment offset Xa corresponds to +F+f0.FIG. 2( d) is a side view illustration of a patterned top layer L2 beingoffset by an amount −F+f0 from a patterned bottom layer L1 in accordancewith one embodiment of the present invention. In one embodiment offsetXb corresponds to −F+f0. FIG. 2( e) is a side view illustration of apatterned top layer L2 being offset by an amount +F+f0+E from apatterned bottom layer L1 in accordance with one embodiment of thepresent invention. FIG. 2( f) is a side view illustration of a patternedtop layer L2 being offset by an amount −F+f0+E from a patterned bottomlayer L1 in accordance with one embodiment of the present invention.

In general, an error offset E in one direction, for example along theX-axis, may be determined by analyzing at least the measured spectra (orany type of measured signals) obtained from four targets A, B, C, and Deach having offsets between two patterned layers, such as offsets Xathrough Xd. This analysis is performed without comparing any of thespectra to a known or reference spectra (or signal) from a sample targethaving a known overlay error. In other words, the error determinationtechniques of the present invention do not require a calibrationoperation.

FIG. 3( a) is a flow diagram illustrating a procedure 300 fordetermining overlay in accordance with one embodiment of the presentinvention. In this example, four targets A, B, C, and D are used whichare designed to have offsets Xa through Xd as described above. That is,target A is designed with offset Xa=+F+f0; target B with offsetXb=−F+f0; target C with offset Xc=+F−f0; and target D with offsetXd=−F−f0.

Initially, an incident radiation beam is directed towards each of thefour targets A, B, C, and D to measure four spectra S_(A), S_(B), S_(C),and S_(D) from the four targets in operations 302 a through 302 d,respectively. Operations 302 a through 302 d may be carried outsequentially or simultaneously depending on the measurement system'scapabilities. The incident beam may be any suitable form ofelectromagnetic radiation, such as laser, light emitting diode (LED), orbroadband radiation.

Although the scatterometry techniques of the present invention aredescribed as utilizing measured spectra from a plurality of targets, anysuitable type of measurable signal obtained from an overlay target maybe used to practice the techniques of the present invention. Examplesignals include, but are not limited to, any type of spectroscopicellipsometry or reflectometry signals, including: Ψ, Δ, Rs (complexreflectivity of the s polarization), Rp (complex reflectivity of the ppolarization), Rs (|r_(s)|²), Rp (|r_(p)|²), R (unpolarizedreflectivity), α (spectroscopic “alpha” signal), β (spectroscopic “beta”signal), and functions of these parameters, such as tan(Ψ), cos(Δ),((Rs−Rp)/(Rs+Rp)), etc. The signals could alternatively or additionallybe measured as a function of incidence angle, detection angle,polarization, azimuthal angle of incidence, detection azimuthal angle,angular distribution, phase, or wavelength or a combination of more thanone of these parameters. The signals could also be a characterization ofa combination of signals, such as an average value of a plurality of anyof the above described ellipsometry and/or reflectometery signal types.The signals may alternatively take the form of a characteristic of oneor more image signal(s), such an intensity value(s) or a combination(e.g., average or addition) of intensity values. Other embodiments mayuse monochromatic or laser light sources where at least one of thesignals may be obtained at a single wavelength instead of at multiplewavelengths.

Examples of optical systems and methods for measuring scatterometrysignals to determine overlay may be found in (1) U.S. patentapplication, having U.S. Pat. No. 09/849,622, filed 4 May 2001, entitled“METHOD AND SYSTEMS FOR LITHOGRAPHY PROCESS CONTROL”, by Lakkapragada,Suresh, et al. and (2) U.S. patent application, having application Ser.No. 09/833,084, filed 10 Apr. 2001, entitled “PERIODIC PATTERNS ANDTECHNIQUE TO CONTROL MISALIGNMENT”, by Abdulhalim, et al. Theseapplications are herein incorporated by reference in their entirety.Further embodiments of suitable measurement systems and their use fordetermining overlay error are further described below.

After a spectra or signal is obtained from each target, spectrumS_(B)(−F+f0) is then subtracted from spectrum S_(A)(+F+f0), and spectrumS_(D)(−F−f0) is subtracted from spectrum S_(C)(+F−f0) to form twodifference spectra D1 and D2 in operations 304 a and 304 b,respectively. Next, a difference spectrum property P1 is obtained fromthe difference spectra D1 and a difference spectrum property P2 isobtained from the difference spectrum D2 in operations 306 a and 306 b,respectively. The difference spectra properties P1 and P2 are generallyobtained from any suitable characteristic of the obtained differencespectra D1 and D2. The difference spectra properties P1 and P2 may alsoeach simply be a point on the each difference spectra D1 or D2 at aparticular wavelength. By way of other examples, difference spectraproperties P1 and P2 may be the result of an integration of averaging ofthe difference signal, equal an average of the SE alpha signal, equal aweighted average which accounts for instrument sensitivity, noise orsignal sensitivity to overlay.

After difference spectra properties P1 and P2 are obtained, the overlayerror E may then be calculated directly from the difference spectraproperties P1 and P2 in operation 308. In one embodiment, a linearapproximation is performed based on the difference spectra properties P1and P2 to determine the overlay error E, while in another technique thedifference spectra properties P1 and P2 are used to approximate a sinewave function or other periodic function which is then used to determinethe overlay error E. One linear regression technique is illustratedbelow with respect to FIG. 3( b). In one example, the overlay result maybe obtained by a statistical calculation (e.g. averaging or weightedaveraging) of overlay results obtained from properties of multiplewavelengths or multiple wavelength ranges.

In a variation of this implementation, if all four targets have the samecharacteristics, such as pitch P, thin film characteristics, structuresize and composition, except for the offsets, and assuming that Xa andXb are opposite in sign and have the same order of magnitude and if Xais the same sign as Xc and Xb is the same sign as Xd, an estimate of theoverlay error E present between structures within the interlayer targetscan be calculated as follows using a linear approximation based on thedifference spectra properties P1 and P2:E′=((S _(C) −S _(D))*(Xa+Xb)/2−(S _(A) −S _(B))*(Xc+Xd)/2)/((S _(A) −S_(B))−(S _(A) −S _(D)))orE′=(P2*(Xa+Xb)/2+P1(Xc+Xd)/2)/(P1−P2)where the difference spectra properties P1 and P2 are generally oppositein sign for overlay errors E<f0. If(Xa−Xb)=(Xc−Xd) and E=0, thenP1=−1*P2.

Alternatively, if the same values for F and f0 are used for designingeach target offset Xa, Xb, Xc, and Xd, thenE′=(f0*P2+f0*P1)/(P1−P2).

The targets may be used to determine overlay of structures located atleast partially in more than one layer, but could also be used todetermine overlay of structures located substantially in a single layer.In other embodiments, the offsets may have the same sign.

FIG. 3( b) shows a graphical representation of the linear approach fordetermining the overlay error E in accordance with one embodiment of thepresent invention. As shown, the positive portion of the y axis shows achange in the difference spectra property P1 as a function of f0+E andthe negative portion of the y axis shows a change in the differencespectra as a function of −f0+E. As described above, the differencespectra properties P1 and P2 are obtained from the difference spectra D1and D2.

The overlay error E may be obtained by analyzing the two points (+f0+E,P1) and (−f0+E, P2). The overlay error E may be determined in oneapproach by performing a linear approximation with the two obtaineddifference spectra properties P1 and P2. Note that there are two pointson the graph where E is zero, while other portions of the graph are afunction of the overlay error E and f0. If the offsets are chosencarefully so as to be in the linear region, then the slope of thepositive portion of the graph (P1/(+f0+E)) should equal the slope of thenegative portion of the graph (P2/(−f0+E). Thus, the overlay error isgiven by E=f0*(P1+P2)/(P1−P2).

According to an implementation of the invention, if there are twotargets with offsets +F and −F that are equal in magnitude but oppositein sign and no other overlay errors, then the 0th diffraction orderscatterometry SE or reflectometry spectra are substantially identicalfrom these two targets (to a good approximation) and the differencesignal between the spectra corresponding to +F and −F is zero. Ofcourse, any property of the difference signal is also zero. If onedeliberately breaks the symmetry (artificially induces an overlay error)by designing an additional offset +f), then the difference signal D1 isno longer zero and any suitable difference spectra property follows thesame relationship as for an overlay error E. Similarly one can designanother set of overlay targets with an additional offset −f0. Thus, theoverlay error may be determined using a property of the differencesignals D1 (+F+f0, −F+f)) and D2 (+F−f0, −F−f0), and accordingly noseparate calibration step is required.

It should be understood that when the overlay error E is calculated fromthe spectra signals, it may be an estimate of actual overlay error. Thecalculated overlay error E may be denoted as the overlay error (E), oran estimate of the overlay error (E′).

If the pitch between structures is relatively large then the abovedescribed linear approximation techniques generally work well. However,when the pitch is relatively small then additional targets may beproduced on the sample to improve the accuracy of the overlaymeasurements. The number of targets and corresponding scatterometrytechniques which are used depend on the particular materials of thetarget and the scatterometry signal type implemented, among otherfactors. The number of targets can be determined experimentally or bywell known modeling methods. In one embodiment, two additionalinterlayer targets (denoted targets “H” and “J”) are produced on thesample, with corresponding offsets Xh and Xj. Upon being illuminated byincident radiation, the targets H and J produce corresponding diffractedcomponents, which can serve as a basis for determination of anadditional difference signal D3 and difference spectra property P3. Thisproperty P3 may be analyzed in connection with the difference spectraproperties P1 and P2 to refine the determination of the overlay E toinclude non-linear corrections or measurements of the errors introducedby using a linear approximation.

In one target implementation, each of the targets A, B, C, and Dcomprises a grating structure Ga1 having periodic structures with aperiod Ta1 disposed at least partially within the first layer and agrating structure Ga2 having periodic structures with a period Ta2disposed at least partially within the second layer (e.g., the target ofFIG. 2 c or 2 d). Generally, a target could be any periodic structurelike a particular device pattern repeated a number of times. One or moreof the gratings Ga1 and/or Ga2 may be formed from device-like (e.g.,design rule based) or process robust (e.g., low variability undervariable process conditions). The first layer period Ta1 and the secondlayer period Ta2 could be identical or different as well (in thesimplest case Ta1=n*Ta2 or Ta2=n*Ta1, where n is an integer), and theoffsets Xa, Xb, Xc, and Xd are each produced by offsetting thestructures with the period Ta1 of the grating structure Ga1 with respectto the structures with the period Ta2 of the grating structure Ga2 bythe sum of a first distance F and a second distance f0, wherein thesecond distance f0 has a smaller absolute value than the first distanceF.

In another target embodiment, the composite periodic structurecomprising Ga1 and Ga2 is periodic with a period Ta, and it is possibleto describe both Ga1 and Ga2 in terms of the period Ta, with Ga1possibly having a complex structure (complex unit cell with multiplecomponents) and Ga2 also possibly having a different complex structure(complex unit cell with multiple components). For example, a unit cellmay include a set of closely spaced line segments adjacent to a largeflat area. This unit cell is repeated to form either or both grating Ga1or Ga2. Gratings Ga1 and Ga2 may have the same or different unit cells.Additionally, the unit cells of Ga1 may be a rational or integer numberof the units cells of grating Ga2, or visa versa. Gratings Ga1 and/orGa2 may also be designed to be similar to the critical devicefeatures—i.e. share one or more of the device characteristics such aspitch, line width, etc. Scatterometry overlay targets designed to besimilar to the device features may provide advantages by processing moresimilarly to the device features, including reflecting thepattern-dependent overlay effects such as pattern-placement error.

One alternative embodiment to the linear approximation methods discussedabove is to treat the scatterometry overlay signal as a periodicfunction and use phase detection methods to determine the overlay error(herein referred to as a phase scatterometry approach). This embodimentmay be preferred in some circumstances, depending on variables that mayinclude scatterometry overlay target pitch, scatterometry overlay targetdesign, scatterometry overlay (SCOL) target materials, the measuredscatterometry signal, and the like.

The overlay error may be extracted from measuring multiple SCOL targetswith pre-programmed additional built-in overlay offsets. (One example ofthe preprogrammed offsets could be Xa, Xb, Xc, and Xd as discussed aboveand in FIG. 1). The number of targets measured may be two, three, four,or more than four, or may vary between different overlay measurementlocations. For the phase methods, it may be advantageous for the offsetsto be evenly distributed throughout the period, with differencescorresponding to the period divided by the number of targets (e.g.Xa−Xc=Xc−Xb=Xb−Xd=Xd−Xa+P=P/4 for 4 targets for one direction).Alternatively, the offsets could be designed to distributed unevenlythroughout the period which may be advantageous when used with somephase-detection algorithms.

A scatterometry signal (as a function of the wavelength or incidentangle, for example) is acquired from each of the required SCOL targets.This signal is generally a periodic and even function of overlay error,for the case where the offsets are measured from a symmetry position ofone of the L1 or L2 patterns. A phase detection (or phase retrieval,phase extraction, or phase determination) algorithm utilizes theseproperties of the signals.

The measured signal is represented by a set of even periodic functionswith a corresponding number of free parameters (one of these freeparameters is the overlay error itself). For example, each measuredsignal may be represented by a Fourier series expansion having anynumber of terms consistent with the number of target measured. Thenumber of terms depends on the number of targets measured, scatterometrysignals properties, target properties, and information requried. In aFourier series having three terms, a measured signal may be representedby:K+lcos((2π/P) (V_(i)+E))+mcos((4π/P)(V_(i)+E))where k is a cnstant; l is an amplitude of the firts harmonic; m is theamptitude of the second harmonic; Vi represents the predefined offset; Pis the period; and E the overlay error. The number of targets measuredis to be greater or equal to the cummulative number of free unknownparameters in the chosen function. In the above three term example,there are four unknown; k, l, m, and E where the period is 360 degreesor 2π radians. Therefore, four targets may be used to solve for the fourunkowns which include overlay E.

When several (two or more) scatterometry overlay (SCOL) targets (withdifferent pre-programmed offsets) are placed in the immediate vicinityof each other (within 0 to 250 microns, for example), the overlay errormay be assumed to be the same for all these targets. Each of the otherfree parameters can either vary or not vary from one SCOL targetlocation to the other one (within the field and/or across the wafer).(Overlay is assumed to vary between different overlay measurementlocations in the stepper field or across the wafer). Alternatively,these free parameters (or some of them) may either vary or not varybetween X- and Y-SCOL target orientations. Based on the informationrequired, the measurement accuracy required and on whether some freeparameters are not varying location-to-location and/or between X- andY-orientations, the total number of SCOL targets per overlay measurementlocation and total number of SCOL targets to be measured per fieldand/or per wafer is determined.

An example of a phase algorithm approach to determining overlay errorfrom scatterometry signals from multiple targets is to treat thedependence of the scatterometry signals on overlay error as a periodicfunction. In this case the programmed offsets of the multiple targetsare treated as initial phase offsets and the overlay error is treated asan additional phase. The overlay error can then be determined usingwell-known phase determination or phase retrieval methods. Well knownphase retrieval methods that may include quadrature, 3-bucket, and4-bucket phase retrieval algorithms can be used to determine overlayerror. These phase retrieval methods are listed as examples only and arenot meant to limit the scope of the invention. Phase detection methodsare well known and are commonly used in diverse areas such ascommunications, interferometry, nuclear magnetic resonance, electroniccircuits, to list a few examples. In another embodiment, a combinationof linear, non-linear, and phase retrieval algorithms may be employed todetermine the overlay error.

Certain conditions are preferably met with implementation of the abovedescribed techniques. The measurement areas are substantially identicalin all aspects except for the offsets, e.g., +F+f0, −F+f0, +F−f0, and−F−f0. This is likely accomplished by placing the targets within about100 microns or less of each other and by choosing targets which arerelatively-robust to the process (i.e. they have similar or lesssensitivity to process variation as the device features). In practice,on production wafers, the profiles may deviate from identical fordifferent offsets if the topography from the lower pattern layer(s) andthe upper layer changes in response to interacting with this topography.A difference or error signal between the two targets with differentoffsets is relatively independent of profile variation of the overlaytarget segments and to film variation as long as the profiles are commonto the different targets. This is the substantial equivalent of commonmode rejection of the parts of the signal that are determined by theprofile and the films and the optics. The technique is also preferablyrobust to the range of process variation encountered in a typicalmanufacturing process. The signal differences due to the overlay errorare also preferably larger than the signal differences due to othersources of process variation between the nearby scatterometry overlaytargets (including mask errors).

If in a particular implementation the targets include structures groupedto exhibit the characteristics of lines, then a separate set of targetsmay be required for X and Y overlay measurements. If the overlay targetsare composed of 2-dimensional structures (as seen from a top down view),then it may be possible to use one set of targets to get both X and Yoverlay information. For oblique scatterometry, according to a specificimplementation, it may be advantageous to rotate the orientation of thewafer with respect to the optical scattering plane to measure thedifferent X and Y overlay errors. For true normal incidence, it may bepossible to get both X and Y overlay information from the differentpolarizations without rotating the wafer or the optics.

Cartesian coordinates provide a convenient frame of reference formeasuring overlay within a sample, with the x-y plane being locatedwithin, or substantially parallel with, a layer of the sample, and withthe z axis being substantially perpendicular to the layers of thesample. The Cartesian coordinate system could be fixed with respect tothe sample or could be rotated to reduce the complexity of themeasurements. For example, overlay occurring diagonally across thesample but within a single layer could be described as two-dimensionalx-y overlay in a Cartesian system with the x-y axes substantiallyparallel with the sides of a rectangular sample or stepper field. Thatsame diagonal overlay could be measured along a single axis, however, byrotating the x-y axes such that the x axis is parallel with thedirection of the diagonal overlay.

In one embodiment, targets include more than one predefined offset,possibly between different sets of structures located in two layers, orpossibly between different sets of structures located in more than twolayers. In a general case, a target may include an indefinite number oflayers, with all or some of these layers having structures producingpredefined offsets. In a particular implementation, the structures inone or more underlying patterned layers of a target cause changes in theshape or topography of one or more upper layers (disposed above theunderlying patterned layer(s)). In this implementation, the one or moreupper layers may be substantially or partially opaque or absorbing, andat least part of the diffraction signal may arise from the topography ofan upper layer, the topography arising at least in part from theunderlying patterned layer.

According to one embodiment, structures included in a target may beorganized in various configurations and shapes, including, for example,lines, grids, rectangles, squares, curved lines, curved shapes, circles,cylindrical shapes, conical shapes or combinations of the foregoing.Such configurations of structures may be disposed at various locationswithin the target, and may describe various angles with respect to theelectromagnetic radiation incident on the target. For example, the setsof structures could be organized as a set of parallel linesperpendicular to the direction of propagation of a collimated set ofradiation rays or of a beam incident on the target. In another case, thestructures organized as a set of parallel lines could be disposed at anacute angle with respect to the incident radiation, possibly at an angleof 45 degrees. Such a configuration could be advantageous byfacilitating determination of overlay in both x and y directions,thereby reducing the need for additional overlay patterns ormeasurements.

Alternatively, the incident radiation could be directed to besubstantially parallel to at least some of the parallel lines comprisingthe structures or defining the structures. This technique allows one toperform x and y overlay measurements without rotating the sample.

1. Scatterometry System Embodiments and Uses of Same

Several of the techniques of the present invention may be implementedusing any suitable combination of software and/or hardware system. Forexample, the techniques may be implemented within an overlay metrologytool. Preferably, such metrology tool is integrated with a computersystem which implements many of the operations of this invention. Suchcomposite system preferably includes at least a scatterometry module forobtaining scatterometry signals of the overlay targets, and a processorconfigured to analyze the obtained scatterometry signals to therebydetermine overlay error within such targets. At a minimum, thescatterometry module will usually include (i) a source of illuminationoriented to direct radiation onto a specified location of the sample;and (ii) one or more detectors oriented to detect a scatterometry signalwhich has been scattered by the sample.

At least a portion of the techniques of the present invention may alsobe implemented in an overlay metrology system as an additional overlaymeasurement capability which complements an overlay measurement systemor sub-system based on image analysis such as one used for conventionalbox-in-box or frame-in-frame overlay targets or other imaging typeoverlay measurement structures. Examples of apparatus which combineimaging-based overlay metrology and scatterometry-based overlay aredescribed in the above referenced Provisional Application No.60/498,524, which is incorporated here by reference. Several embodimentsof such a combinational system are described further with respect toFIGS. 11 d through 11 f. Overlay data from imaging overlay measurementsand scatterometry overlay measurements may be combined for various usesincluding: calculating the overlay correctables, calculating otheroverlay corrections, calculating overlay errors at other locations onthe wafer. More use cases for combinations of imaging overlay metrologyand scatterometry overlay metrology are also described in abovereferenced Provisional Application No. 60/498,524 and are describedfurther below.

Regardless of the system's configuration, it may employ one or morememories or memory modules configured to store data, programinstructions for the general-purpose inspection operations and/or theinventive techniques described herein. The program instructions maycontrol the operation of an operating system and/or one or moreapplications. The memory or memories may also be configured to storescatterometry data obtained from the targets and overlay error resultsand optionally other overlay measurement data.

Because such information and program instructions may be employed toimplement the systems/methods described herein, embodiments of thepresent invention relates to machine readable media that include programinstructions, state information, etc. for performing various operationsdescribed herein. Examples of machine-readable media include, but arenot limited to, magnetic media such as hard disks, floppy disks, andmagnetic tape; optical media such as CD-ROM disks; magneto-optical mediasuch as floptical disks; and hardware devices that are speciallyconfigured to store and perform program instructions, such as read-onlymemory devices (ROM) and random access memory (RAM). The invention mayalso be embodied in a carrier wave traveling over an appropriate mediumsuch as airwaves, optical lines, electric lines, etc. Examples ofprogram instructions include both machine code, such as produced by acompiler, and files containing higher level code that may be executed bythe computer using an interpreter.

Several of the system embodiments described below are mainly describedand illustrated with respect to a scatterometry module or components forobtaining spectra (or other measurable signals) from a plurality oftargets, while the processor and memory are not shown. Additionally,several of the systems are described herein with respect to the abovedescribed linear scatterometry approach. Of course, any suitablescatterometry approach, such as the phase approach may be utilized.

Imaging Metrology Systems in Which the Numerical Aperture is Optimizedfor Measurement of Scattering Structures

FIG. 4 is a diagrammatic representation of a microscopic imaging system.As shown, the imaging system 400 includes a beam generator 402 forproducing an incident beam 403 of electromagnetic radiation, a beamsplitter 404 for directing the incident beam 405 towards the sample 408.Typically, the incident beam is focused onto the sample by an objectivelens 406. An output beam 409 is then emitted from the sample in responseto the incident beam and passed through the beam splitter 404 throughrelay lens 410 onto imager or camera 412. The camera 412 generates animage of the sample based on the output beam 409.

The system 400 also includes a processor and one or more memory 414which are configured to control the various components, such as the beamgenerator 402, objective lens 406, and camera 412. The processor andmemory are also configured to analyze the detected output beam or imageimplementing the various scatterometry techniques described above.

Traditionally, such imaging systems (such as those used for overlay)have selected numerical apertures (NA's) (e.g., via objective lens 406)to optimize image resolution and to minimize optical aberrations.Selection of NA is typically performed in order to derive the overlayinformation from the variation in intensity over a single target (suchas a box-in-box target ) from the geometrical properties of the image.

Conventional imaging systems have relied upon high numerical apertures(NA's), such as 0.7 to 0.9, but doing so results in an expensive opticalsystem which is sensitive to vibration, depth of focus, and opticalaberrations. These issues reduce the achievable precision and cause ameasurement error referred to as “tool induced shift” or TIS.

Scatterometry systems may take measurements at multiple sitessequentially in order to measure both x and y overlay and to eliminateeffects due to variations in other sample parameters, such as filmthickness. This type of measurement process results in significantlyslower operation of the scatterometry tool relative to conventionaloverlay techniques.

In one embodiment of the present invention, the illumination and imagingNA's of an imaging optical system are chosen to optimize the performanceof the instrument on scattering structures by ensuring that only thezero'th diffraction order is collected. One may take advantage of thefact that there exist performance advantages for certain metrology orinspection tasks pertaining to periodic structures when only the zeroorder diffraction is collected by the detection system. Under thiscondition, only the specular reflection is collected. Since the outputwhich is scattered out of the specular is not collected and thenonspecular output may be more sensitive to aberrations, collection ofonly the specular output tends to minimize effects caused by opticsaberrations. This condition also will result in a tool which will beoptimized for relative photometric measurements of multiple sites in thefield of view as described further below. Very low TIS may also beachieved, as compared to conventional imaging systems. Much higherthroughput may also be achieved than with conventional scatterometrysystems.

Choosing the illumination and imaging NA's for a particular imagingsystem is based on the particular configuration of such system. If wenow consider the simplest imaging system in which the numerical apertureNA of illumination and collection are the same and the incident beam innormal to the sample surface, then the condition of “zero orderdiffraction only” can be met if:nλ>2dNA, where n=1.where d is the pitch of the structures of the targets being imaged. Thiscan be restated in terms of the numerical apertures of illuminationNA_(i) and collection NA_(c) of the imaging system as:nλ=d(NA _(i) +NA _(c))This equation indicates that if we are able to constrain the numericalaperture of the illumination system, we can relax the constraint on thenumerical aperture of the collection optics, which may be advantageousunder certain conditions. Thus, the spectral range may be restricted towavelengths greater than twice the product of the pitch and the NA.Under realistic conditions the scattered radiation beam will be wider(more divergent) than the illumination beam. Under realisticcircumstances however, infinitely periodic gratings are not imaged andso the above equations become approximations and the diffracted planewaves become somewhat divergent. So it may be preferable to include amargin of safety in the constraint and require that:nλ≧2dNA(1+ε), where n=1 and ε is small, typically less than 0.5.

As an example, for an NA 0.4 imaging system, wavelengths may berestricted to values greater than 0.8 times the largest pitch, whichdoes not seem to be an unreasonable constraint. For periodic structureshaving features of design rule 70 nm and below, the densest structureswith pitches as low as 200 nm does not constrain the spectral range ofimaging systems with operating wavelengths equal to about 200 nm orlonger, while more isolated features with pitches as large as 500 nm arepreferably measured with wavelengths longer than 400 nm.

It is preferable to account for these constraints when designing animaging spectrometer for metrology and inspection applications. A limiton the spatial resolution of the imaging system is the numericalaperture of the system. It is advantageous to achieve the highestspatial resolution so as to be able to shrink to a minimum the size ofmetrology structures and conserve valuable wafer real estate. Restated,this allows minimization of proximity effects or “crosstalk” betweenadjacent features in the field of view of the imaging spectrometer.Therefore, the highest possible NA is achieved while meeting theconstraint that only the zero order diffraction is collected by thedetection system.

Another interesting outcome of this constraint is that the highestpossible overlay spatial resolution may be achieved without everresolving the features under test. This may have further advantages asit should ensure that problematic aliasing phenomena are avoided in theimaging system. In a preferred embodiment, an architecture is providedin which the spectral band pass can be easily modified or selected bythe measurement system or algorithm based on the largest pitch in thefeature under test (e.g., as the systems in FIGS. 5A through 5Ddescribed further below). Alternatively, the NA of either illuminationor collection could be easily modified, depending on the largest pitchin the feature under test. Alternatively, all of these embodiments maybe implemented within a single system.

FIGS. 5A through 5E illustrate four embodiments of microscopic imagingsystems having a numerical aperture (NA) optimized for scatteringcharacteristics. As shown in FIG. 5A, the system 500 may have componentswhich operate like the same named components of the system in FIG. 4.The system 500 further includes a wavelength selection device 520 forselecting a particular wavelength. The wavelength selection deviceallows light of different wavelengths to be sent selectively to one ormore detectors. A wide variety of well known spectroscopic filteringtechniques may be employed to modify the spectra band, includingselecting from a set of band pass interference filters, continuouslyvarying bandpass interference filters, grating based spectrometers,acousto-optic tunable filters, to name a few. The wavelength selectiondevice 520 is positioned within the incident beam path. The system 500may also include a polarizer control device 522 for causing the incidentbeam to be in a particular polarization state and a polarizationanalyzer 524 for analyzing or separating out the polar components of thecollected beam.

The system 530 of FIG. 5B is similar to the system 500 of FIG. 5A,except a wavelength modulation device 532 is used in place of awavelength selection device. The system 540 of FIG. 5C is similar to thesystem 500 of FIG. 5A, except the wavelength selection device 542 ispositioned in the output beam path. The system 550 of FIG. 5D is similarto the system 500 of FIG. 5C, except a wavelength modulation device 552is used in place of a wavelength selection device. The wavelengthmodulation device operates by modulating the intensity of differentwavelengths in different temporal patterns such as different sinusoidalfrequencies. The most common examples of such a device areinterferometers which can be controlled by changing one or more opticalpath lengths in the wavelength modulation device 532 itself (e.g., aninterferometric system, such as in a Michelson, Fabry-Perot, or Sagnacinterferometers). The spectral information may be derived from theresulting signal with a transform analysis like a Fourier transform orHadamard transform, for example.

FIG. 5E is a top view representation of an imaging spectrometer,multiple site field-of-view example in accordance with one embodiment ofthe present invention. In one implementation, spectra from one or morepixels in each dotted box are averaged to create a spectrum for each ofthe four measurement targets. Alternatively, spectra from one or morepixels located only in a central region of each dotted box are averagedtogether. Size and spacing of lines in the illustrated targets areexaggerated for emphasis. There is an area where the lines of layer 2are disposed above the lines of layer 1 for at least part of the target.The signals for this area are detected as scatterometry overlay signals.Another example of the targets is shown in FIG. 11 a.

FIG. 5( f) is a diagrammatic representation of a fixed, discrete channeloptical system in accordance with a fifth embodiment of the presentinvention. In this embodiment, the system includes a mirror havingspectroscopic apertures 562. That is, the mirror is reflected, exceptfor a plurality of apertures which let the light from the sample passthrough in particular spatial portions. FIG. 5( g) is a diagrammaticrepresentation of the aperture mirror of FIG. 5( f) in accordance withone embodiment of the present invention. As shown, the mirror 572includes four etched apertures 574. The apertures 574 are etched withina mirror reflective substrate 572. In one implementation, the lightwhich corresponds to each center portion of each target pass through themirror to separate detectors, e.g., fiber pickoffs to spectrometers 564.The remaining portion of the target image, excluding the central imageportions for each target, is reflected by mirror 562 to a camera. FIG.5( h) is a top view representation of an imaging spectrometer, multiplesite field of view example with missing aperture components (that aresent to spectrometers) in accordance with one embodiment of the presentinvention. As shown, the camera image of the targets contain missingportions 582 which signals are sent to spectrometers, instead of acamera.

The NA of any of the above described systems may be selected to ensurethat only the zero'th diffraction order is collected in any suitablemanner. In one proposed operational embodiment:

-   -   1. Two or more sites of differing characteristics are located in        the field of view of the imaging system.    -   2. Images are captured over one or more spectral ranges.    -   3. For each measurement site in the field of view, all or some        of the pixels determined to be within that site are summed or        otherwise combined to characterize the photometric properties of        that site in that spectral range.    -   4. Step 3 is repeated for each spectral range.    -   5. The results for each site over each spectral range are        processed to determine the properties of the sample. For        example, the above described spectral analysis techniques (i.e.,        F+f0) are used on the obtained spectra for each target.    -   6. Steps 1 though 5 are repeated for the plurality of        measurement sites desired across the wafer.

While this example technique describes sequentially capturing imagesover different spectral regions, this could be accomplishedsimultaneously using a system of wavelength dependent beam splitters,filters, and/or mirrors. Alternatively, the same could be affected byusing a device such as a Sagnac interferometer which captures multipleimages at different optical path differences, these being used to deriveinformation equivalent to images taken over different spectral ranges.

Scatterometric Overlay Using Filters

Conventional imaging overlay tools have a high magnification and smallfield of view. Inspection for gross patterning defects is either donemanually on a microscope or automatically on a separate macro inspectiontool. A low magnification overlay tool unfortunately requires multiplesteps or tools, some of which are manual.

In one embodiment, a low magnification microscope with a mechanism forselecting one or more wavelength ranges is provided. This tool alsopreferably uses one or more broadband sources with filters, withmultiple sources covering different wavelength ranges, variable filters,etc. FIG. 6 is a diagrammatic representation of a system 600 forselecting one or more wavelength ranges in accordance with oneembodiment of the present invention. As shown, the system 600 includes abroadband source 602 for generating a multiple wavelength incident lightbeam 604 towards sample 606. A multiple wavelength output beam 608 isscattered from the sample 606 in response to the incident beam 604. Thesystem 600 also includes a filter 610 for selectively passing a portionof the output beam 611 based on wavelength to camera 612. In oneimplementation, the filter is configurable to pass particular colors,such as red, green, blue, or yellow. The camera is operable to generatean image based on the filtered output beam 611.

Measurements of overlay are taken by moving to a location on the samplewhere one or more of the targets in a target set are in the field ofview of the microscope. An image is acquired and the intensity from someor all of the pixels in the image, which includes each individualtarget, are averaged or summed to give an intensity value for the targetat a particular setting of the filter. In one embodiment, the filter isadjusted so as to give a maximum difference between targets. This maysubsequently be normalized with respect to a reference surface, to thenumber of pixels summed, or corrected by a map of the illuminationuniformity within the field of view. The sample or optics may then bemoved until all of the necessary targets in a target set are measured.The overlay value is then determined using the intensity values in anyof the above described scatterometry techniques, such as the linearapproach by:P1=(Ia−Ib) and P2=(Ic−Id)And:Overlay=f0*(P2+P1)/(P2−P1)

This process could be repeated over multiple wavelength ranges toimprove accuracy, precision, and robustness, wherein the wavelengthswhich result in the best contrast are used for the scatterometryanalysis.

Because the magnification is low and the field of view is large comparedto a typical imaging overlay tool and because an image of the area ofthe sample is colleted, unlike a conventional reflectometer orellipsometer, analysis of the image may be used to detect other types ofprocessing problems by analyzing the image. For example, if the wrongreticle has been used for one or more processing steps, the image wouldbe materially different. If the resist thickness were incorrect, thebrightness or contrast of the image may be effected. If resist streakingwere present, variation of brightness or contrast over the image may bedetected. In CMP (chemical mechanical polishing) processes, processingerrors such as over-polish, under-polish, etc. could be similarlydetected.

In this embodiment, multiple scatterometry targets can be measuredsimultaneously, increasing the measurement speed. Additionally,processing errors or changes in processing conditions other than overlaycan be detected without the need for a separate inspection tool.

Simultaneous, Multi-Angle Scatterometry

Techniques of obtaining scatterometric measurements may include thetheta or 2-theta approach, in which scattering intensity from a gratingor other repeating structure is measured at a plurality of angles bymaking multiple, sequential measurements. As the sample rotates throughan angle of theta, the detector is generally rotated through 2-theta.Alternatively, the angles of the incident beam and detector system maybe changed simultaneously. Use of the 2-theta approach is very slow,since multiple measurements are typically made. Use of multiple anglescanning scatterometry, such as a scanning angle of incidence system,requires mechanics which can accurately scan through a precise range ofangles.

In a specific embodiment of the present invention, techniques andapparatus for simultaneous, multi-angle scatterometry are provided.Unlike the 2-theta approach, measurements are made with an apparatuswhich permits scattering intensity to be simultaneously determined formany angles. This technique is far faster than the 2-theta approach.

In order to implement this approach, an optical apparatus such as thatshown in U.S. Pat. No. 5,166,752 by Spanier et al could be used. Thispatent is herein incorporated by reference in its entirety. In thispatent, a multi-angle ellipsometer is shown in, for example, FIGS. 3 and4 of the Spanier et al. patent. FIG. 7 is a diagrammatic representationof a simultaneous, multiple angle of incidence ellipsometer 700. Asshown, the ellipsometer includes a source generator (e.g., components702, 706, 708, 710, and 712) for directing polarized light onto thesurface of sample 714, detection optics (e.g., components 718 through724) for handling and detecting the output beam emitted from the sample,and a detector 726 for generating a signal related to the polarizationstate of the light reflected from the sample. The source generatorincludes a light source 702, a polarizer 708, a compensator 710 with avariable aperture, and a focusing lens system 712 simultaneouslydirecting polarized light from a single beam of light from the lightsource onto the sample's surface at different angles of incidence. Thesource generator may also include an optional optical narrow bandfilter.

The lens system 712 has an effective aperture to focal length ratio forfocusing the light on the sample 714 with angles of incidence which varyover a range of angles of at least one or two degrees. In a particularembodiment, the range of angles of incidence is about 30 degrees. Largerangles could be employed for directing rays at the sample 714.

The focusing lens system 712 focuses the polarized light which may befrom a He-Ne laser for example, down to a single small spot or point onthe sample 714. The different incident rays may have widely varyingangles of incidence which are focused on a single, small spot on thesample 714. Thus, the light directed on the small spot on sample 714contains rays at many angles of incidence above and below the angle ofincidence of the central ray through the focusing lens. Each one of theincoming rays is reflected at an angle equal to its angle of incidencewith the polarization state of each of the rays being altered by thatreflection. A detector array 726 is employed to detect a plurality ofrays reflected from the sample 714 individually over different, narrowranges of angles of incidence to simply and quickly obtain data at aplurality of angles of incidence.

The output beam emitted from the sample 714 is directed through outputlens 716, interchangeable aperture 718, polarization analyzer 720, andan optional alternate filter 724 onto detector array 726. The diameter dof the lenses 712 and 716 corresponds to their effective diameter. Inthe illustrated embodiment the lenses 712 and 716 each have a diameter dof 18 mm and a focal length 1 of 34 mm. Other effective lens diametersand focal lengths could be employed so long as a range of angles ofincidence, preferably at least 30 degrees, is provided. The lensdiameter and focal length are chosen with a view toward maximizing thenumber of angles of incidence of the light beams which strike the sample714. In an alternative embodiment, light is transmitted through thesample rather than reflected from a surface of the sample.

The refocusing lens or lenses 716 directs the reflected (transmitted)light toward the detector array 726. However, a refocusing lens need notbe employed as the reflected (transmitted) light could be made todirectly impinge upon an array of detectors. It is preferable that thelenses 712 and 716 do not themselves alter the polarization state of thelight.

The detector array 726 may be a linear, multiple element detectorwherein each of the detector elements can detect a narrow range ofangles of incidence of the rays that illuminate the sample. In thedisclosed embodiment, the array 726 is a solid-state photosensitivedetector array wherein the separate detector elements are all integratedon one circuit chip. Particularly, the detector elements comprise alinear array of photodiodes. While integrated on a single circuit chip,the individual photodiodes can function as separate detectors. Thelinear array of the disclosed embodiment comprises 128 detector elementsarranged in a row to provide data for 128 different angles of incidencewhere the full array is illuminated by the reflected (transmitted)light. The number of individual detector elements could be more or lessthan that in the disclosed embodiment and the detector elements need notbe integrated on a single chip but could be discrete detectors. By usinga plurality of detector elements, it is possible to simultaneouslydetect the light reflected from the surface (or transmitted through thesample) for each of a plurality of different angles of incidence. It isalso possible with the invention to employ a smaller number of detectorelements which could be sequentially moved to mechanically scan thereflected (transmitted) rays for detection but this technique wouldrequire more time and could be less accurate, depending upon positioningaccuracy.

The physical size of each of the detector elements is preferably lessthan the expanse of the reflected rays so that each element detects onlya certain narrow range of angles of incidence on the illuminating side.The output of each of the detectors is used in a conventional manner aswith real time computer techniques (e.g., via analyzer 720) to generatedata in terms of Δ and Ψ for each of those narrow ranges of angles ofincidence. The data may then be used in any of the above describedscatterometry approaches, as well as interpreted in a conventionalmanner. The linear array preferably runs in the plane of the opticalsystem. In the disclosed embodiment, the long axis of the lineardetector array 726 lies in the plane of incidence of the central ray andperpendicular to the central ray for detecting the maximum number ofincidence angles. Alternatively, the compensator 710 could be placedafter the sample 714 before analyzer 720 instead of, or in addition to,being located before the sample 714.

Such an ellipsometer could be used to illuminate a scatterometry targetsimultaneously over a range of angles, and an intensity of the scatteredlight is measured over a range of angles simultaneously with an arraydetector or the like. The signals acquired with a simultaneous,multi-angle system may be analyzed with a self-calibrating multi-targetmethod such as the linear or phase-based methods described above.

By collecting data from the intensities measured at those angles, theparameters of the grating or other target can also be determined. Forexample, the data can be compared against theoretical models of dataderived from techniques such as those mentioned by U.S. Pat. No.6,590,656, issued Jul. 8, 2003, entitled “SPECTROSCOPIC SCATTEROMETERSYSTEM” by Xu et al, which patent is herein incorporated by reference inits entirety. The data can also be compared to theoretical modelsderived from techniques such as those mentioned by U.S. patentapplication, having application Ser. No. 09/833,084, filed 10 Apr. 2001,entitled “PERIODIC PATTERNS AND TECHNIQUE TO CONTROL MISALIGNMENT”, byAbdulhalim, et al., which application is herein incorporated byreference in its entirety. This comparison can then be used to extractstructure or target parameters from a database based on such comparison.

The model can also be adjusted based on such comparison. For instance,when the measured data significantly differs from the theoretical data,the model which was used to generate the theoretical data may then beadjusted so as to generate a more accurate value.

The data can be pre-generated and stored in libraries, or generated inreal time during analysis. It is also possible, for techniques likescatterometric overlay, to directly compare measured spectra associatedwith various targets. Such differential measurements can then be used todetermine overlay misregistration.

It would also be possible to perform this technique with a beam profilereflectometer such as that described in U.S. Pat. No. 4,999,014, issued12 Mar. 1991, entitled “METHOD AND APPARATUS FOR MEASURING THICKNESS OFTHIN FILMS” by Gold et al., which patent is incorporated herein byreference in its entirety.

An alternative embodiment of a simultaneous, multi-angle opticalapparatus suitable for measuring scatterometry signals for overlay is anOptical Fourier Transform instrument described in SPIE Vol. 4299,pp279-290, (2001) by Obein, et al, which is incorporated herein byreference in its entirety. An implementation of this optical concept isthe EZ-Contrast by ELDIM of Hérouville Saint Clair, France. Apolarizer-may be used to control the polarization of the incident beam.A polarizing element may be used to analyze the polarization of thescattered radiation before it reaches the detector or CCD. The resultingscatterometry signals may be analyzed with the linear algorithm or aphase detection algorithm described herein. The Optical FourierTransform instrument may be configured to operate with a singlewavelength, multiple wavelengths operating in parallel or in series, orwith a Fourier transform modulation on the incident radiation.

Simultaneous Ellipsometry and Reflectometry

A system for employing a combination of ellipsometers and reflectometersmay be employed to improve the accuracy of scatterometric measurementsof overlay. In one embodiment, two or more ellipsometers are utilized asscatterometers to measure overlay. One of more of these ellipsometerscould be spectroscopic ellipsometers. In another embodiment, two or morereflectometers are utilized as scatterometers to measure overlay. One ofmore of these reflectometers could be polarized reflectometers.Alternatively, a combination of one or more ellipsometers and one ormore reflectometers are utilized as scatterometers to measure overlay.

Measurements can be performed serially (with each tool performingmeasurements at different times), in parallel (with all tools performingmeasurements substantially-simultaneously, or in any other arrangement(e.g., at least two but less than all of the tools performingmeasurements substantially-simultaneously).

In any of the implementations described herein, various tools mayperform measurements at different angles of incidence, including nearnormally and obliquely, or both normal and oblique. That is, two or moreof the following systems may be used together to achieve both a nearnormal incidence and one or more oblique angles: a spectroscopic nearnormal incidence reflectometer, a spectroscopic near normal incidencepolarized reflectometer, a spectroscopic near normal incidence polarizeddifferential reflectometer, an oblique incidence spectroscopicellipsometer, and a spectroscopic oblique incidence polarizeddifferential reflectometer.

In a specific implementation, at least two tools perform scatterometricmeasurements at substantially the same angle of incidence but fromdifferent directions. For instance, a first tool would be used forscatterometric measurements in the x direction, and a second tool wouldbe used for scatterometric measurements in the y direction. Such asystem could eliminate certain common scattered signals, with acorresponding increase in accuracy of measurements, and provide asymmetric configuration.

An advantage of employing a combination of such tools in scatterometricdetermination of overlay is that the accuracy of the measurements couldbe increased. Another advantage in using more than one tool andperforming measurements at more than one angle (or point) of incidenceis to help separate effects affecting the medium of interest (e.g., filmeffects) from overlay. For example, ellipsometer signals or polarizationdependent signals from optical systems operating at normal ornear-normal incidence have lower sensitivity to film thickness than atmore oblique angles of incidence but have significant sensitivity to theoverlay of scatterometry overlay targets. A further advantage is thatcombinations of ellipsometers and reflectometers already exist incurrent inspection tools. Another advantage of employing a combinationof scatterometers configured to perform scatterometry measurementssubstantially in parallel on different targets or different targetsections could be to reduce the total time required for measurement.Another advantage of a parallel measurement system could be to increasethe signal acquisition time for each scatterometry overlay target andimprove the measurement precision.

Scatterometric Overlay Determination Using FT processing

A system for scatterometric measurement of overlay using fouriertransform (FT) processing may also be utilized. In one embodiment, aninterferometer is employed to modulate substantially all wavelengths ofa broadband source, and the scattered radiation is detected with a CCDcamera. Substantially all wavelengths of the modulation band arerecorded for each pixel, or for groups of pixels. As the interferometersteps through the modulation band, a spectroscopic image of thescattered signal is produced.

The resulting spectroscopic image may have a relatively large field ofview. For example, the image may include several multiple targets. Thespectroscopic image could be processed on a pixel-by-pixel basis toaccurately determine overlay while eliminating extraneous effects (e.g.,film effects). Alternatively, processing could be performed using groupsof pixels to improve speed and decrease processing resources. Forexample, a group of pixels from each target may be analyzed using any ofthe above described scatterometry processes. In a linear scatterometryapproach, the images for each corresponding pair of targets issubtracted to obtain difference images D1 and D2. A characteristic, suchas average intensity, of each difference signal is then obtained toresult in P1 and P2, which are then used to determine the overlay error.

In a specific implementation, a Michelson interferometer is used to stepthrough a wavelength modulation band. Alternatively, a Linnikinterferometer, or any other interferometer, could be employed. For eachposition of the mirror, a CCD camera records the scattered signalintercepted in the field of view of the camera. The detected signals maythen be digitized and stored on a pixel-by-pixel basis, or as groups ofpixels. The magnitude of the steps is generally proportional with theaccuracy of the overlay measurement. The speed of the camera (e.g., thenumber of fields per second that the camera can capture) is typicallyproportional with the speed of the measurement. Once the modulation bandis spanned, the signal recorded for each pixel (or group of pixels) maybe used as a basis for a discrete fourier transformation (or DFT). TheDFT provides a spectral profile for each pixel (or group of pixels).Alternatively, the Fast Fourier Transform (FFT), Hadamard transform, orother known transform methods could be applied. Similarly, convolutionor other mathematical methods could be used to determine the spectralprofile. This spectral profile for each target may then be used in anyof the scatterometry overlay techniques described above. Overlaydetermination can then be performed with increased accuracy.

Multiple Tunable Lasers

A system which has a combination of tunable lasers may be utilized toimprove the accuracy of scatterometric measurements of overlay incombination with measurements performed by various configurations ofellipsometers and reflectometers. The tunable lasers provide radiationincident on the surface of interest. In one embodiment, scatterometricoverlay measurements are performed using targets disposed in at leastone layer of the design under consideration, and the tunable lasersprovide radiation beams incident on the targets at multiple lasersettings (e.g., at multiple wavelengths).

The measured signals may then be averaged together before or afterprocessing. In one example linear scatterometry approach, measuredradiation beams are obtained from targets A, B, C, and D. Two differencesignals D1 and D2 from each pair of targets may then be obtained atmultiple tunable laser settings. The signals measured from each targetfor each tunable laser setting may be averaged together prior toobtaining the difference signals D1 and D2. Alternatively, each set ofdifferences signals for D1 and D2 may be averaged together to obtain asingle average difference signal D1 and D2. Properties P1 and P2 of thedifference signals D1 and D2 (e.g., integration) may then be obtained.In an alternative embodiment, multiple properties P1 and P2 are obtainedfor the different configurations of the tunable laser (without averagingthe measured signals or the difference signals D1 and D2) and theresults are averaged for each signal P1 and P2. The overlay error maythen be obtained based on the signals P1 and P2 as described above.Alternatively, a phase scatterometry approach may be used by obtainingmeasured signals at multiple wavelengths from a plurality of targets.

Similarly, one ore more light emitting diodes covering one or morewavelength ranges might be used.

Scatterometric Overlay Determination Using Spatial Filtering

One embodiment expands on the above described embodiment forScatterometric Overlay Determination using FT Processing.

A system for scatterometric measurement of overlay using FT processingin connection with spatial filtering is provided. More particularly, thesignal reflected by at least one scatterometry target is selectivelyfiltered spatially to only process particular signal components.

In the above described embodiment for Scatterometric OverlayDetermination using FT Processing, an interferometer is employed tomodulate substantially all wavelengths of a broadband source, and thescattered radiation is detected with a detector, such as a CCD camera.Substantially all wavelengths may then be recorded for each pixel, orfor groups of pixels. As the interferometer steps through the modulationband, a spectroscopic image of any spatial portion of the scatteredsignal is produced. In the present example, where the scattered signalcorresponding to a complete image (or a portion of an image) iscollected, only a portion of the signal corresponding to a single lineof pixels is retained. Alternatively, a portion of the signalcorresponding to a plurality of pixel lines, but less than the wholeimage, is collected. Such a selective collection of the scattered signalcan be achieved by spatially filtering the signal to only retainhorizontal, vertical or oblique stripes of the signal corresponding torows of pixels in the detector or CCD camera. Alternatively, a larger,more complete portion of the scattered signal could be collected at theCCD camera, but the information corresponding to undesirable rows ofpixels (e.g., an edge of a target or a border between two targets) maybe discarded subsequent to the collection.

The spectroscopic image corresponding to the retained signal may then beprocessed on a pixel-by-pixel basis to accurately determine overlaywhile eliminating extraneous effects (e.g., film effects). For instance,particular spatial portions of the scattered radiation may be blocked toremove particular frequency and/or phase information. Alternatively,processing could be performed using groups of pixels to improve speedand decrease processing resources. This embodiment provides higher SNR(signal to noise) over conventional processing methods.

In one implementation of the invention, any of the above describedtechniques to determine overlay in reference to the ScatterometricOverlay Determination using FT Processing embodiment may be used.

Compared to the embodiment for Scatterometric Overlay Determinationusing FT Processing, aspects of the embodiment for ScatterometricOverlay Determination Using Spatial Filtering may improve the processingspeed and the throughput, while decreasing processing resources.

Examples of Spectroscopic Ellipsometers and Spectroscopic Reflectometers

FIG. 8 is a schematic view of a spectroscopic scatterometer system 800,in accordance with one embodiment of the present invention. The system800 combines the features of a spectroscopic ellipsometer 802 andspectroscopic reflectometer 804, each of which may be used for measuringoverlay of a grating structure 806 disposed on a substrate or wafer 808.The grating structure 806, which is shown in a somewhat simplifiedformat in the Figure, may be widely varied. The grating structure 806may, for example, correspond to any of those grating structuresdescribed herein. Both the spectroscopic ellipsometer 802 andspectroscopic reflectometer 804 may utilize a stage 810, which is usedfor moving the substrate 808 in the horizontal xy directions as well asthe vertical z direction. The stage may also rotate or tilt thesubstrate. In operation, the stage 810 moves the substrate 808 so thatthe grating structure 806 can be measured by the spectroscopicellipsometer 802 and/or the spectroscopic reflectometer 804.

The spectroscopic ellipsometer 802 and spectroscopic reflectometer 804also utilize one or more broadband radiation sources 812. By way ofexample, the light source 812 may supply electromagnetic radiationhaving wavelengths in the range of at least 230 to 800 nm. Examples ofbroadband light sources include deuterium discharge lamps, xenon arclamps, tungsten filament lamps, quartz halogen lamps, and light emittingdiodes (LEDs). Alternatively, one or more laser radiation sources may beused instead of or in combination with the broadband light source. Inthe case where the signal is collected at only one or a few wavelengths,the system may not be considered a spectroscopic ellipsometer but may bereferred to as a single wavelength (or multi-wavelength) ellipsometer.

In the spectroscopic reflectometer 804, a lens 814 collects and directsradiation from source 812 to a beam splitter 816, which reflects part ofthe incoming beam towards the focus lens 818, which focuses theradiation onto the substrate 808 in the vicinity of the gratingstructure 806. The light reflected by the substrate 808 is collected bythe lens 818, passes through the beam splitter 816 to a spectrometer820.

The spectral components are detected and signals representing suchcomponents are supplied to the computer 822, which computes the overlayin any of the manners described above.

In the spectroscopic ellipsometer 802, the light source 812 supplieslight through a fiber optic cable 824, which randomizes the polarizationand creates a uniform light source for illuminating the substrate 808.Upon emerging from the fiber 824, the radiation passes through anoptical illuminator 826 that may include a slit aperture and a focuslens (not shown). The light emerging from the illuminator 826 ispolarized by a polarizer 828 to produce a polarized sampling beam 830illuminating the substrate 808. The radiation emerging from the samplingbeam 830 reflects off of the substrate 808 and passes through ananalyzer 832 to a spectrometer 834. The spectral components of thereflected radiation are detected and signals representing suchcomponents are supplied to the computer 822, which computes the overlayin any of the manners described above.

In the spectroscopic ellipsometer 802, either the polarizer 828 or theanalyzer 832 or both may include a waveplate, also known as compensatoror retarder (not shown). The waveplate changes the relative phasebetween two polarizations so as to change linearly polarized light toelliptically polarized light or vice versa.

In order to collect more information about the interaction of theincident polarized light 830 with the sample, it may be desirable tomodulate the polarization state of the light or modulate thepolarization sensitivity of the analyzer or both. Typically this is doneby rotating an optical element within the polarizer and/or analyzer. Apolarizing element within the polarizer or analyzer may be rotated, or,if at least one of those assemblies contains a waveplate, the waveplatemay be rotated. The rotation may be controlled by the computer 822 in amanner known to those skilled in the art. Although the use of a rotatingelement may work well, it may limit the system 802. As should beappreciated, the use of rotating elements may be slow, and because thereare moving parts they tend to be less reliable.

In accordance with one embodiment, therefore, the polarizer 828 isconfigured to include a polarization modulator 836, such as photoelasticmodulator (PEM), in order to produce a fast and reliable spectroscopicellipsometer. The polarization modulator replaces the rotatingwaveplate. The polarization modulator 836 is an optical element thatperforms the same function as a rotating waveplate, but without thecostly speed and reliability problems. The polarization modulator 836allows electrical modulation of the phase of the light withoutmechanically rotating any optical components. Modulation frequencies ashigh as 100 kHz are readily attainable.

In an alternative embodiment, the analyzer 832 is configured to includea polarization modulator such as a PEM (Photoelastic Modulator) that canbe modulated electrically. In yet another embodiment, both the polarizerand analyzer contain polarization modulators, such as PEMs, that aremodulated at different frequencies.

Because the polarization modulator 836 can modulate at such a highfrequency, the polarization modulator 836 may be used to perform varioustechniques, which would otherwise be too slow. For example, thedifference between the polarized reflectivity of two structures may beobtained. To do this, a PEM may be combined with an acoustic opticalmodulator (AOM), where the AOM rapidly moves between the two structureswhile modulating the polarization state at a different (but related,such as multiple or submultiple) frequency. Signals at the sum and thedifference of the PEM and AOM modulation frequencies contain usefulinformation and can be detected with high signal-to-noise by synchronousdetection. Alternatively the AOM on the incident beam could be used incombination with a PEM in the analyzer.

Although not shown, the rotating waveplate may also be replaced by apolarization modulator in other types of scatterometric systems as forexample a polarization sensitive reflectometer.

Another optical system that may be used for scatterometry overlaymeasurements is a differential reflectometer or differentialellipsometer for detecting the +/−1 diffraction orders as described inthe above referenced U.S. patent application Ser. No. 09/833,084 byAbdulhalim et al. which is incorporated herein by reference. One of thesignals that may be analyzed is differential intensityDS=(R₊₁−R⁻¹)/(R₊₁+R⁻¹). The signals may be measured from multiplescatterometry overlay targets with various offsets as described above.The resulting scatterometry signals may be analyzed with the linearalgorithm or a phase detection algorithm described herein to determinethe overlay.

Scatterometric Overlay Database

One aspect of the present invention provides a database ofscatterometric overlay information that may be utilized forscatterometic overlay determination.

In one implementation, one or more database are provided which includeone or more libraries of overlay information. The database informationis then used in overlay measurements.

In one implementation, the libraries are compiled using predeterminedtest patterns with artificially-induced overlay. Alternatively, thelibraries are produced using layer misregistrations programmed into thestepper. In another embodiment, the overlay that is induced orprogrammed has a progressive characteristic, varying within a particularrange.

The information stored into the database may include overlay dataregarding the actual overlay printed on the wafer, as induced via thetest pattern or by the stepper. Alternatively or additionally, thisinformation is obtained from overlay actually measured on samples. Thedatabase may further store scatterometry measurement records associatedwith the overlay data. Such scatterometry measurement records may beobtained by performing actual scatterometric measurements of the overlaydata. The database may also include information regarding materials,process conditions, optical parameters, and other relevant data. Thedatabase information may be further enhanced by interpolation and otherpreprocessing.

The scatterometry database information may be utilized to improve theaccuracy and speed of overlay measurements by retrieving scatterometrydata associated with particular scatterometric measurements and processconditions recorded during actual measurements. In one implementation,theoretical overlay data which is generated using models and varioustarget and optics configurations is recorded in a database. When overlayis measured on a particular set of targets, the measured overlay maythen be matched to a particular theoretical overlay value. Targetcharacteristics, for example, associated with the matching theoreticalvalue may then be obtained.

Dynamic selection of a measurement algorithm or methods may also beprovided based on database lookups. A further implementation utilizesthe database to calibrate scatterometric overlay measurement toolsbefore or during production line measurements.

Alternative Systems for Performing Scatterometry

According to various embodiments of the invention, acquisition of thespectra A through D (and of additional spectra if present) is performedusing an optical apparatus that may comprise any of the following or anycombination of the following apparatus: an imaging reflectometer, animaging spectroscopic reflectometer, a polarized spectroscopic imagingreflectometer, a scanning reflectometer system, a system with two ormore reflectometers capable of parallel data acquisition, a system withtwo or more spectroscopic reflectometers capable of parallel dataacquisition, a system with two or more polarized spectroscopicreflectometers capable of parallel data acquisition, a system with twoor more polarized spectroscopic reflectometers capable of serial dataacquisition without moving the wafer stage or moving any opticalelements or the reflectometer stage, imaging spectrometers, imagingsystem with wavelength filter, imaging system with long-pass wavelengthfilter, imaging system with short-pass wavelength filter, imaging systemwithout wavelength filter, interferometric imaging system (e.g. Linnikmicroscope, e.g. Linnik microscope as implemented in the KLA-Tencoroverlay measurements tools models 5100, 5200, 5300, Archer10, etc.available from KLA-Tencor of San Jose, Calif.), imaging ellipsometer,imaging spectroscopic ellipsometer, a scanning ellipsometer system, asystem with two or more ellipsometers capable of parallel dataacquisition, a system with two or more ellipsometers capable of serialdata acquisition without moving the wafer stage or moving any opticalelements or the ellipsometer stage, a Michelson interfereometer, aMach-Zehnder interferometer, a Sagnac interferometer, a scanning angleof incidence system, a scanning azimuth angle system.

Additionally, the optical modules of any of the above described multipleoptical module systems may one or more optical elements in common. Forinstance, a system with two or more polarized spectroscopicreflectometers capable of parallel data acquisition which share at leastone optical element, with separate spectrometers or detectors for theradiation scattered from different targets (targets Ax and Cx, or Ax andAy for example). Likewise, a system with two or more spectroscopicellipsometers capable of parallel data acquisition may have at least oneoptical element in common, with separate spectrometers or detectors forthe radiation scattered from different targets (targets Ax and Cx, or Axand Ay for example). By way of another example, a system with two ormore ellipsometers capable of parallel data acquisition may have atleast one optical element in common, with separate spectrometers ordetectors for the radiation scattered from different targets (targets Axand Cx, or Ax and Ay for example).

Several embodiments of an interferometer based imaging spectrometer, aswell as other types of imaging spectrometers such as filter based or the“push broom” approach, are described in U.S. Patent, having U.S. Pat.No. 5,835,214, issued 10 Nov. 1998, entitled “METHOD AND APPARATUS FORSPECTRAL ANALYSIS OF IMAGES”, by Cabib et al. System and Methodembodiments for film thickness mapping with spectral imaging aredescribed in U.S. Patent, having U.S. Pat. No. 5,856,871, issued 5 Jan.1999, entitled “FILM THICKNESS MAPPING USING INTERFEROMETRIC SPECTRALIMAGING”, by Cabib et al. An alternative architecture for spectralimaging based on LED illumination is described in U.S. Patent, havingU.S. Pat. No. 6,142,629, issued 7 Nov. 2000, entitled “SPECTRAL IMAGINGUSING ILLUMINATION OF PRESELECTED SPECTRAL CONTENT”, by Adel et al.These patents are incorporated herein by reference in their entirety forall purposes.

The imaging spectrometer or reflectometer used for acquisition of thespectra A through D from the four targets (and of additional spectra ifpresent) according to an embodiment of the invention may be of theFourier transform imaging spectrometer type as is well understood bythose skilled in the art. The imaging system of the Fourier transformimaging spectrometer should be capable of separating (resolving) thereflected or scattered light signals from the different targets (orsections of a compound scatterometry overlay target). Alternatively theimaging spectrometer or reflectometer used for acquisition ofscatterometry overlay signals may use a two-dimension detector where oneaxis contains the spatial information from the different scatterometryoverlay targets (or sections of a compound scatterometry overlay target)and the other detector axis contains spectrally resolved informationfrom light spectroscopically separated with a prism system ordiffraction grating system, for example or a system that is acombination of a prism and a grating. The illumination radiation may bewavelength selected prior to incidence on the target.

The spectra A through D obtained from the four targets (and additionalspectra if present) detected in the imaging spectrometers, imagingreflectometers, or any of the other optical systems identified above inconnection with various embodiments of the present invention may beunpolarized or selectively polarized. One or more of the unpolarizedlight or one or more of the polarization components of the reflected orscattered light from the targets may be detected with the imagingspectrometer or the imaging reflectometer.

In various implementations, separate detection systems may be used toseparately or simultaneously record one or more of the following lightsignals: unpolarized reflected light, polarized light with the electricfield substantially parallel to one major symmetry axis of one layer ofthe scatterometry overlay targets, polarized light with the electricfield substantially perpendicular to one major symmetry axis of onelayer of the scatterometry overlay targets, polarized light with theelectric field at an angle to one major symmetry axis of one layer ofthe scatterometry overlay targets, right-hand circularly polarizedradiation, left-hand circularly polarized radiation, and/or acombination of two or more of the previously listed polarization states.A separate detector system may be used to simultaneously record thesignal from part of the light source for the purposes of light noisemonitoring, and/or light level control, and/or light noise subtractionor normalization.

Various possible implementations of various embodiments of the presentinvention are illustrated in co-pending U.S. Provisional Application No.60/449,496, filed 22 Feb. 2003, entitled METHOD AND SYSTEM FORDETERMINING OVERLAY ERROR BASED ON SCATTEROMETRY SIGNALS ACQUIRED FROMMULTIPLE OVERLAY MEASUREMENT PATTERNS, by Walter D. Mieher et al. Thisprovisional application is herein incorporated by reference in itsentirety.

In one embodiment, each of the four targets (and additional targets ifpresent) is illuminated by radiation produced by an optical system. Theoptical system may take the form of, among others, an optical source, alensing system, a focusing system, a beam shaping system, and/or adirecting system. In one embodiment, the radiation illuminating at leastone of the targets is shaped as a radiation beam, with a relativelynarrow beam cross section. In a particular implementation, the beam is alaser beam. The radiation illuminating the targets interacts withstructures comprised within the targets and produces diffractedradiation components corresponding to each target and denoted as S_(A),S_(B), S_(C), and S_(D) (and additional signals if present). In oneembodiment, the illuminating beam is a broadband polarized beam having abroad spectral range as is commonly used in spectroscopic ellipsometry.In one implementation, a focusing system may include one or morefocusing mirrors.

2. Scatterometry Overlay Technique Alternatives:

Several related techniques are described in the above related co-pendingU.S. Provisional Applications. These related techniques may be easilyintegrated with the techniques described herein.

In one embodiment of the invention, the targets (or compoundscatterometry target sections) with different programmed offsets +/−Fand +/−f0 as described above, or +/−F or other similar targetcombinations, are grouped together to enable simultaneous signalacquisition. In one implementation targets are arranged in a line toenable data acquisition while scanning the wafer or some or all of theoptics in one direction along the array of scatterometry overlaytargets. Arranging the targets in a linear array may also enable use ofan imaging spectrometer or reflectometer, where one detector axisseparates the signals from the different targets (or target sections)and the other detector axis detects the spectral information. In thiscase the imaging system images a linear or cylindrical image of thelinear target array into the prism or grating system. The imagingspectrometer or imaging reflectometer may contain an array of two ormore lenses (known to those skilled in the art as a lenslet array) toseparate and direct the reflected or scattered light from differenttargets or target sections.

In one embodiment, the primary offset F is optimized to provide largeror maximum sensitivity to overlay errors. For instance, an offset Fequal to ¼ of the pitch of the target provides high overlay sensitivitysince it is half-way in-between the two symmetry points where overlayerror sensitivity is minimum. The secondary offset f0 may be chosen suchthat the f0 is outside the region of interest for overlay measurements,such as equal to or beyond the specification limits, but it should notcause the uncertainty of the overlay measurement to allow the error thatan out-of-spec measurement can be interpreted as within specifications.Nevertheless, this is not a limitation on the range of f0. A large f0may decrease the accuracy of the overlay measurements for overlay errorsE between −f0 and +f0. For overlay errors E larger than |f0|, theaccuracy of the overlay measurement may be reduced due to extrapolationbeyond the region −f0 to +f0 and the accuracy of the linearapproximation may also be reduced.

Overlay measurements are most commonly done at or near the four cornersof the stepper field (sometimes with an additional measurement near thecenter of the field) in 5 to 25 fields per wafer in semiconductormanufacturing processes. For a system of four targets used to determineoverlay in the x direction and four targets used to determine overlay inthe y direction, according to an embodiment of the present invention, atotal of 8*4*5=160 measurements of scatterometry overlay targets may beused to determine the two dimensional overlay for a common overlaymeasurement sampling plan. More measurements may be conducted for moredetailed sampling plans.

According to another embodiment of the invention, a total of six targets(three for x and three for y, for example) can be used to determine twodimensional overlay for the sample. This may facilitate furthersimplification of the overlay metrology process, reduction in processingresources, and decrease of the time used in the metrology process. Inyet other implementations, additional targets or additional pairs oftargets may be produced on the sample and used in a substantiallysimilar manner with that described herein for determination of overlaybased on scatterometry, but adjusted for the increased number of targetsand corresponding number of diffracted radiation components. Themathematical methods for determination of the overlay error E can besimilarly adjusted to exploit the availability of increased informationprovided by such additional targets or additional pairs, including bypossibly accounting for higher order approximation terms in the formulafor the overlay error E.

Scatterometric Overlay Determination With Limited Refocusing

To improve the accuracy of scatterometry overlay determination, morethan one measurement is preferably carried out. One implementationutilizes a plurality of scatterometry overlay targets, and for eachtarget, the system makes one scatterometric measurement of overlay.Another implementation utilizes a single scatterometric target, or asingle scatterometric target area that comprises multiple targetsub-regions, and more than one scatterometric overlay measurement isperformed for that target or target area. In yet another embodiment, aplurality of targets or target regions are used, and more than onemeasurement is performed for some or all of the targets or targetregions.

Conventionally, the optical system is refocused for each individualmeasurement. This, however, can consume a lot of time thus decreasingthe processing speed of the system. For example, each focus sequence maytake between 0.01 and 1 seconds, and each wafer may include between 30to 70 sites with each site consisting of 8 targets. Using these numbers,refocusing may take up to as much as 560 seconds for each wafer.Considering there are typically 100s and 1000s of wafers to be inspectedthis number may be further increased to a completely unacceptable level.

In accordance with one embodiment of the present invention, therefore,multiple scatterometry overlay measurements are performed with limitedoptical refocusing in order to increase the processing speed andthroughput of the system. By limited optical refocusing, it is generallymeant that at least some new measurements are performed withoutrefocusing the optical system, i.e., multiple measurements are made withthe same focus setting. For instance, the optical system may beinitialized with a focus setting that is optimized for a plurality ofscatterometric measurements that will be performed, and no furtherrefocusing takes place during these individual scatterometricmeasurements. The optimized focus setting may be found once for theentire wafer, or it may be found periodically. When periodic, the focussetting may be established at preset increments of time duringinspection (e.g., every 30 seconds), for a particular location on thewafer (e.g., every 2×2 cm2 of wafer), for a particular characteristic ofthe target (e.g., similar line widths and spacing) and the like.

In one embodiment, the wafer includes a plurality of focus zones. Eachof the focus zones is initialized with a focus setting that is optimizedfor all of scatterometric measurements that will be performed within thefocus zone. Refocusing does not occur between individual scatterometricmeasurements inside the focus zone. As such, each target within thefocus zone is measured with the same optimized focus setting. Any numberof focus zones may be used.

The configuration of the focus zones may be widely varied. In oneimplementation, the focus zones correspond to a portion of the wafer. Byway of example, the wafer may be broken up into plurality of radialfocus zones emanating at the center of the wafer and working outwards,or into a plurality of angular focus zones, which separate the waferinto multiple quadrants. In another implementation, the focus zonescorrespond to a particular set of targets as for example, the targets atthe corners of each semiconductor device. In another implementation, thefocus zone corresponds to a particular target area that includes aplurality of targets (see for example 9A). In another implementation,the focus zones correspond to a particular target sub-region within thetarget areas (as for example the x or y directed group of targets shownin FIG. 9B). In yet another implementation, the focus zone correspondsto a particular sub region within the target itself.

A method of determining overlay will now be described. The methodgenerally includes optimizing the focus setting of a first zone. Themethod also includes performing a first set of measurements on aplurality of targets within the first zone. Each of the targets withinthe first zone is measured using the optimized focus setting of thefirst zone. That is, a first target is measured, and thereafter a secondtarget is measured without refocusing the optical system. Any number oftargets can be measured in this manner. The method further includesoptimizing the focus setting of a second zone. The method additionallyincludes performing a second set of measurements on a plurality oftargets within the second zone. Each of the targets within the secondzone is measured using the optimized focus setting of the second zone.That is, a first target is measured, and thereafter a second target ismeasured without refocusing the optical system. Any number of targetscan be measured in this manner.

In one example of this method, the first and second zones may representdifferent target areas that include a plurality of targets (See FIG.9A). In this example, each of the targets are located in close proximityto one another and therefore it can be assumed that variations in focusfrom one target to the next are minimal. The method generally includesoptimizing the focus setting in the target area, and thereaftermeasuring each of the targets in the target area with the optimizedfocus setting. For example, the first target is measured, thereafter theadjacent target is measured, and so on without ever refocusing theoptical system. When a first target area is measured, the system mayrepeat these steps on a second target area, as for example, a targetarea located at a different corner of the device.

In another example of this method, the first and second zones mayrepresent sub regions with a target area that includes a plurality oftargets. The sub regions may for example represent different targetorientations (See FIG. 9B). The method generally includes optimizing thefocus setting in the first sub regions (e.g., targets along the x axis),and thereafter measuring each of the targets in the sub region with theoptimized focus setting. For example, the first target is measured,thereafter the adjacent target is measured, and so on without everrefocusing the optical system. When the first sub region is measured,the method continues by optimizing the focus setting in the second subregions (e.g., targets along the y axis), and thereafter measuring eachof the targets in the sub region with the optimized focus setting. Forexample, the first target is measured, thereafter the adjacent target ismeasured, and so on without ever refocusing the optical system. Inanother example the system is refocused prior to the measurement on thefirst scatterometry overlay target in an xy scatterometry overlay targetgroup. After the scatterometry signal is measured for the first targetin the xy overlay target group, the rest of the targets may be measuredwithout refocusing. For example, an xy overlay target group comprisesfour scatterometry overlay targets for an overlay error determination inthe x direction and four scatterometry overlay targets for an overlayerror determination in the y direction.

Scatterometric Overlay Determination Using a Line Image

A system for scatterometric measurement of overlay using aone-dimensional line image may also be implemented. This embodimentallows a more efficient collection of light than techniques whichutilize a two dimensional field of view which encompasses an area largerthan the target areas. Additionally, optics may be used in the incidentbeam's path to provide a one dimensional profile for the light incidenton the sample.

FIG. 10 is a diagrammatic top view representation of a system 1000 forobtaining a line image of a plurality of targets 1008 a-1008 d inaccordance with one embodiment of the present invention. As shown, alight source 1002 directs a beam towards cylindrical optics 1004configured to illuminate a one-dimensional (1D) incident line 1006 ofthe targets 1008 a-1008 d. The light source and the incident optics areaffanged so that the 1D incident line strikes at least a portion of allof the four targets. For example, the 1D line is incident on a linethrough the center of the four targets.

Light is then scattered or reflected from the targets in response to theincident line 1006 and some of the reflected light may pass through anoptional slit 1010 to thereby form a 1D output beam having a 1D lineprofile 1012. The 1D output line then may be received by a dispersiveelement 1014, such as a prism or diffraction grating, which spreads orseparates the output beam. In other words, the dispersive element 1014acts to spatially resolve the output beam onto separate detectorelements corresponding to different wavelength ranges or values. Theseparated output beams are then each received by a detector element of2D detector array 1016. This implementation represents an efficientlight delivery and collection mechanism since light is only directed toa narrow band of interest and the light collected is analyzed from thissame narrow band of interest.

In one implementation, the detector array is formed from a plurality ofdetector elements arranged in a 2D array, such as a CCD camera. Onedimension of the detector (e.g., the x direction) may receive separatedoutput beams having different wavelengths, while the second dimension(e.g., the y direction) may receive dispersed output beams havingdifferent positions on the targets. For example, each element of aparticular y direction column of the array 1016 receives a separatedoutput beam having a particular wavelength and corresponding to adifferent position on the targets being imaged, while each element of aparticular x direction row received a separated output beam having adifferent wavelength and a same target position.

Alternatively, the dispersive element 1014 may be omitted and a 1Ddetector may then be used to receive output beams at a plurality of 1Ddetector elements that each correspond to a different target position.In this embodiment, each detector element may average or integrate overdifferent wavelengths and a same target position. In either case,different sets of detector elements may be grouped together ascorresponding to a particular targets. For example, the elements in they direction may be divided into four groups and each group correspondingto a particular one of four targets. An alternative to illuminating asingle incident line is illuminating a larger area but only capturingscattered radiation along a detection line. In another implementation,the cylindrical optics 1004 may be removed from the incident path sothat the incident image is two dimensional. The output beams are thenpassed through cylindrical optics to thereby form a 1D line image forthe detector. A dispersive element with a 2D detector array may also beused as described above. Of course, a 1D detector array may also beimplemented in this implementation.

The image captured by the detectors or camera can be processed at pixellevel to determine overlay, possibly using the FT approach disclosedherein. Once overlay is measured along a particular incidence line, thewafer could be rotated by 90 degrees (or by any arbitrary angle) tomeasure overlay in a different direction. An advantage of the presentinvention is that overlay may be measured in more than one directionusing a single optical system.

Algorithms

Various algorithms and methods for determining overlay may be combinedfor the purposes of refining and cross checking results. Also,pre-existing information (like CD or profile data) may be usefullyintegrated within these techniques.

In one example implementation of a combinational approach, a firstcalculation of overlay is performed according to a first technique (suchas the differential method). A second calculation of overlay is thenperformed according to a second technique (such as a model-basedregression). The results are then combined from the two calculations.The results may be combined in various ways. For example, onecalculation may be used to cross check another. Or one calculation maybe used to provide initial values to speed up the other calculation.Other combinations may also be used.

In a second combinational example, the speed and/or accuracy of anoverlay measurement may be enhanced by making use of other measureddata. For example, film thickness data from the layers making up thetarget may be fed into the algorithm. Providing film thickness or CDdata as an input to the model-based regression program for overlayreduces the number of free parameters or provides better initial guessesfor one or more of the free parameters in the regression thus speedingup the time to result. Such film thickness data could be measured usingan appropriate tool, such as an ellipsometer or reflectometer.Alternatively (or additionally), CD data could be provided from an SCDmeasurement (scatterometry critical dimension or scatterometry profilemeasurement) and used to speed up or improve the accuracy of thescatterometry calculations. Other data from a scatterometry profilemeasurement could be similarly used, such as height or three dimensionalprofile information. Other sources of CD data, like a CD SEM, could beused.

In a specific implementation, a calculation of overlay is performedaccording to a first technique (such as the differential linear methodor phase detection method). A second calculation of the structure of thetarget is then performed according to a second technique (such as amodel-based regression) using the overlay result of the first method.For example, the overlay result of a differential or phase detectionmethod may be used to adjust the model used in the second model-basedregression technique so as to improve model accuracy. For instance, if adifference between the overlay error from the model-based regressiontechnique and the overlay from the differential or phase detectiontechnique is significant (e.g., greater than a predetermined value),then the model is adjusted. This method may speed up or improve thequality of the target structure calculation, for example, and may beadvantageous for determining useful target structure information such asline width of the layer 2 structures, for example. Other combinationsmay also be used.

In yet another embodiment, the difference signal(s) may be calculatedfirst and then the calculation of overlay is performed according to asecond technique (such as a model-based regression). The differencesignal(s) may be used in the second technique, e.g., the model includesdifference signal(s) parameters. The difference signal is generally moresensitive to overlay and less sensitive to other non-overlay propertiesof the targets such as film thickness or feature profiles.

Combined Scatterometry and Imaging Targets and Uses of CombinedScatterometry and Imaging Data

In an alternative implementation, the targets are designed for animaging based overlay metrology application, as well as for the abovedescribed scatterometry analysis. In other words, the scatterometry andimaging target structures are tightly integrated so that scatterometrymay be performed in conjunction with an image based overlay measurement.Preferably, the scatterometry target pairs are symmetrically positionedabout the center of the field of view. If symmetry is preserved in theillumination and collection channels of the imaging system, tool inducedshift will be minimized. By example, Xa and Xa′ are twin (have similarmagnitude but opposite sign offset) targets in the x direction. (Here Xaand Xa′ may correspond to the targets Xa and Xd in FIG. 1). Likewise, Xband Xb′ are opposites. (Here Xb and Xb′ may correspond to the targets Xband Xc in FIG. 1). In the y direction, targets Ya and Ya′ are opposites,while Yb and Yb′ are opposites.

FIG. 11 a is a top view representation of a first combination imagingand scatterometry target embodiment. In this example, the targetarrangement includes a set of four x direction targets for determiningoverlay using scatterometry and a set of four y direction targets fordetermining overlay using scatterometry. The targets are laid out sothat adjacent targets (with respect to the overlay measurementdirection) have an opposite offset. In the illustrated example, targetXa has an opposite offset than target Xa′, and target Xb has an oppositeoffset than target Xb′. Likewise, targets Ya and Ya′ have oppositeoffsets, and targets Yb and Yb′ have opposite offsets. In this example,the targets also include structures which can be used for imaged basedoverlay determination.

In the illustrated example, the target arrangement includes a blackborder structure 1104 on a first layer and a gray cross-shaped structure1102 on a second layer. Using image analysis methods, the center of theblack structure 1104 may then be compared with the center of the graystructure 1102 to determine an overlay error (if any).

Although this set of targets have an overall rectangular shape whichextends longer in the x direction than the y direction, of course, thetargets could have other shapes (e.g., square or any symmetricalpolygon) and/or extend longer in a direction other than x.

In other combinational target arrangements, the imaging structures arelaid out in the center of a symmetrically arranged set of scatterometrytargets. FIG. 11 b is a top view representation of a second combinationimaging and scatterometry target embodiment. As shown, scatterometrytargets are symmetrically arranged around a central image type target1152. In this example, the image type target 1152 is formed fromquadrants of line segments, where each quadrant is either in the x or ydirection. Suitable image type targets and techniques for determiningoverlay with same are described in the following U.S. patents andapplications: (1) U.S. Pat. No. 6,462,818, issued 8 Oct. 2002, entitled“OVERLAY ALIGNMENT MARK DESIGN”, by Bareket, (2) U.S. Pat. No.6,023,338, issued 8 Feb. 2000, entitled “OVERLAY ALIGNMENT MEASUREMENTOF WAFER”, by Bareket, (3) application Ser. No. 09/894,987, filed 27Jun. 2001, entitled “OVERLAY MARKS, METHODS OF OVERLAY MARK DESIGN ANDMETHODS OF OVERLAY MEASUREMENTS”, by Ghinovker et al., and (4) U.S. Pat.No. 6,486,954, issued 26 Nov. 2002, entitled “OVERLAY ALIGNMENTMEASUREMENT MARK” by Levy et al. These patents and applications are allincorporated herein by reference in their entirety.

FIG. 11 c is a top view representation of a third combination imagingand scatterometry target embodiment. This target arrangement hasscatterometry target symmetrically arranged around a box-in-box typetarget 1154. A box-in-box target generally includes a first inner boxformed from a first layer surrounded by a second outer box structureformed in a second layer. The centers of the inner box structures may becompared to the center of the outer box structures to determine overlayerror (if present).

The above targets may be imaged in any suitable manner (e.g., asdescribed in the above referenced patents and applications by Bareket,Ghinovker et al., and Levy et al.) to determine overlay. The targetarrangements may also be simultaneously or sequentially measured withany suitable optical tool as described herein to determine overlay usingscatterometry techniques. In an alternative embodiment, thescatterometry targets may be simultaneously imaged along with theimaging type target structures. The resulting image may be subdividedinto the separate scatterometry targets and then the scatterometrytechniques applied to the image signals for each target (e.g.,intensity).

The image may be obtained at the same time as, or before or after thescatterometry overlay measurements. Imaging overlay techniques may beused on the image. The imaging system may be a high-resolutionmicroscope such as the system in the KLA-Tencor 5300 or Archer overlaymeasurement systems available from KLA-Tencor of San Jose, Calif.Alternatively, the imaging system may be a lower resolution imagingsystem used for other purposes that may include wafer alignment orpattern recognition.

Another use case analyzes overlay measurements where some of the overlaymeasurements on a sample (e.g., wafer or wafer lot) are obtained withimaging overlay metrology techniques and some of the overlaymeasurements are obtained with scatterometry overlay metrologytechniques, which may follow the same or different sampling plans. Inthis general use case, the imaging overlay data may be obtained togetheron the same tool or on a different overlay tool as the scatterometryoverlay data.

One advantage of measuring and analyzing both imaging and scatterometryoverlay in the same wafer or lot is the utilization of the advantages ofboth techniques. For example, imaging overlay can currently be used onsmaller targets than current scatterometry overlay technology.Scatterometry overlay metrology tends to have better performance, suchas better precision and likely better accuracy, than imaging overlaymetrology. Scatterometry overlay metrology tend to have no associatedtool induced shift (TIS), while imaging overlay metrology is associatedwith TIS. The acquisition time of imaging overlay data tends to shorterthan acquiring scatterometry overlay due to the scatterometry targetshaving a larger relative size and the use of multiple targets in thescatterometry approach.

Imaging overlay metrology may be selected for specific targets of awafer and scatterometry overlay metrology for other specific targetsusing any suitable criteria. Any combination of the criteria outlinedbelow may be used to select scatterometry and/or imaging metrology forspecific targets. In one embodiment, scatterometry metrology is used forthe layers which have a tighter overlay budget. That is, scatterometrymetrology is used for targets from the layers which have a low tolerancefor overlay errors, such as the shallow trench isolation to poly layers.Imaging metrology may then be used for the layers which are noncriticalor have looser overlay budgets or constraints.

Additionally, imaging or scatterometry metrology may be selected forparticular targets based on analyzing the trade-offs between performanceversus throughput or wafer real estate. For instance, smaller targetsmay be used in tighter spaces such as in-chip, while larger targets areused larger spaces such as in the scribe lines or streets locatedbetween fields or dies, respectively. In one implantation, largertargets are distributed across the field of the lithography tool in thescribe line, while smaller targets are placed across the field within inthe one or more dies. Scatterometry overlay may be used for the largertargets, e.g., in the scribe lines or streets, while imaging overlay isused for the smaller targets, e.g., that are located in-chip or withinone or more dies. In one implementation, scatterometry metrology is usedfor targets within the scribe line (and/or streets), and imagingmetrology is used for all other targets at other locations. Severalembodiments for placing targets across the field either in-chip or inthe streets or scribe lines are described in detail in co-pending U.S.Provisional Application No. 60/546,546 filed 20 Feb. 2004, entitledAPPARATUS AND METHODS FOR DETERMINING OVERLAY AND USES OF SAME, by MarkGhinovker et al., which application is incorporated herein by referencein its entirety for all purposes. In another implementation, overlay maybe determined on two layer and simultaneous type targets as described inthis provisional application. In one implementation, scatterometrymetrology may be used for simultaneous or single layer targets, whileimaging metrology is used for two layer targets, or visa versa.

Scatterometry metrology may also be performed on particular targets soas to facilitate calibration of the imaging overlay tool. That is,scatterometry overlay is obtained from specific sites, while imagingoverlay is obtained from other sites. When scatterometry overlay differssignificantly from the imaging overlay (more than a predefined value),operating parameters of the imaging tool may then be adjusted andscatterometry and imaging overlay obtained again until the scatterometryand imaging overlay data do not substantially differ (differs less thana predetermined value).

Scatterometry metrology may be associated with limited dynamic range,and accordingly, larger overlay errors may be missed by thescatterometry metrology. Thus, when overlay for a particular set oftargets is expected to exceed the dynamic range limits of scatterometrymetrology (or visa versa), imaging metrology may be used for suchtargets (or visa versa). Additionally, scatterometry may have problemswith highly dense pitch patterns or targets, especially in the polylayer. In this scenario, imaging metrology may be used for highly densetargets that cause a problem for scatterometry, while scatterometry isused for more isolated (less dense) features. Alternatively, imagingoverlay metrology may be found to have problems with density. In thiscase, scatterometry metrology would be used with highly dense features,while imaging is used on isolated features.

In the future targets may be imprinted with a tool other than an imagingphotolithography tool, such as an e-beam direct write lithography toolor a nano-imprint lithography tool. These different tools may havedifferent sampling requirements or modalities. For instance, a tool maynot have an associated field and corresponding sampling. In one example,an e-beam may directly “write” 100's of tiny targets. In this case, animaging metrology tool may be used, while scatterometry metrology isused for targets formed from imaging lithography. In this scenario, asystem which incorporates both imaging and scatterometry metrology ispreferable so that the appropriate metrology may be quickly chosen forthe different tool modalities.

In sum, both scatterometry and imaging overlay data may be usefullycollected on a single sample, such as a wafer or wafer lot. The combinedscatterometry and imaging overlay data may be used in variousapplications. In one application, the scatterometry and imaging overlaydata are both used for lot deposition. When the scatterometry and/or theimaging overlay data is out of specification (e.g., the overlay errorsare higher than a predetermined threshold), it may be determined thatthe current lot is out of specification or is likely to result in actualdevice faults. In this case, the lot may be reworked or thrown out.

In a process excursion use, the scatterometry and imaging overlay datamay be used to determine whether the photolithography tool or processhas deviated out of specification. In other words, the scatterometry andimaging overlay data is used to determine whether there is somethingwrong with the tool or process. In this use case, a special test wafermay be used to check the process or tool. Additionally, other processesand their respective tool's may be assessed via analysis of thescatterometry and imaging overlay data. When the scatterometry andimaging overlay data is out of specification (e.g., greater than apredetermined threshold), it may be determined that a problem hasoccurred in the process or tool and a root cause may then beinvestigated. By way of examples, problems of the lithography mayinclude one or more of the following: resist thickness problems, scanneror stage alignment problems, lens movement alignment problems, focus ordose problems, and lens aberration.

The scatterometry and imaging overlay data may also be used to generatecorrectables for the particular lithography tool (e.g., stepper orscanner tool). In one implementation, the scatterometry and imagingoverlay data are used to determine the dependency between overlay errorand position (e.g., across the field). This dependency may be translatedinto parameters, such as translation, magnification, and rotation, forcorrecting the lithography tool.

Any suitable tool or combination of tools may be used to perform bothimaging and scatterometry overlay. FIG. 11 d illustrates a combinationalimaging and scatterometry system 1160 in accordance with a firstembodiment of the present invention. In this implementation, the imagingoptical assembly 1162 is separate from the scatterometry opticalassembly 1164. In other words, the imaging assembly 1162 is spatiallyseparate from the scatterometry assembly 1164 and both assemblies arestand alone components. In this implementation, the assemblies 1162 and1164 do not share any optical components, but are designed to complimentand collaborate with each other. For instance, overlay data may bepassed between the two devices for implementation of one or more of theabove described techniques on either assembly or on a separate processor(not shown).

The combination system 1160 also includes a stage 1166 for holding thesample thereon. The stage and the optical assemblies move in relation toone another so that the stage can be in a first position under theimaging optical assembly 1162 and in a second position under thescatterometry optical assembly 1164. The stage and/or the opticalassemblies 1162 and 1164 may be coupled to a translational motor.Although a single isolation chamber and stage 1166 are shown for system1160, the imaging and scatterometry assemblies may have their own stageand separate isolation chambers.

Any combination of the above described systems may also be used toobtain scatterometry and imaging overlay data.

FIG. 11 e illustrates a combinational imaging and scatterometry system1170 in accordance with a second embodiment of the present invention. Inthis implementation, the imaging and scatterometry optical assembliesare integrated together. The imaging and scatterometry opticalassemblies may share one or more components. For example, the imagingand scatterometry assemblies may share a same light source. As shown,the combination system 1170 includes an imaging microscope 1172configured for imaging overlay determination and light source 1174 fordirecting any form of optical beam towards a sample on stage 1178 anddetector 1176 for measuring a resulting signal in response to theincident optical beam. For example, the imaging and scatterometryassemblies may share a same light source. The imaging and scatterometryassemblies may also be configured to share data, which may be analyzedin either assembly or by an independent processor (not shown).

FIG. 11 f illustrates a combinational imaging and scatterometry system1180 in accordance with a third embodiment of the present invention.This system 1180 is in the form of a cluster tool. As shown, the system1180 includes a scatterometry module 1182 for obtaining and analyzingone or more scatterometry signals and an imaging module 1186 forperforming imaging overlay determination. The system 1180 also includesa sample handling component 1190 for moving the sample between the twomodules 1182 and 1186. The imaging and scatterometry assemblies may alsobe configured to share data, which may be analyzed in either assembly orby an independent processor (not shown).

The above described systems may contain any suitable components forperforming imaging and scatterometry overlay determination. Forinstance, the imaging optical elements may be similar to the componentsof an Archer system from KLA-Tencor of San Jose, Calif. Thescatterometry optical elements may be arranged like any scatterometrysystem elements described herein.

Mask Alignment During Imprint Lithography

Because the mask and sample are typically in close proximity (separatedby the fluid to be polymerized) during nano-imprint lithography, thepatterned surface of the mask, the fluid, and the patterned sample to bealigned to can be considered to be functionally equivalent to ascatterometry overlay target. The mask contains impression ordepressions arranged in the shape of targets (as well as otherstructures) so that when the mask is pressed into the fluid, animpression that corresponds to the mask target shapes (and otherstructures) is formed in the underlying fluid. Also, a significantportion of the mask is transparent to allow radiation to pass therethrough onto the fluid to thereby make solidify the fluid and itsimpressions that were formed by the mask.

All of the methods, techniques and targets defined for scatterometryoverlay would then be applicable to alignment procedures. In oneembodiment, the measurement instrument projects radiation (preferablylight) through the mask and onto an area of the mask and wafer whichcontains one or more scatterometry overlay targets. For example, thewafer may contain one or more targets on a first layer, while the maskcontains one or more targets which will be used as second layer. Theradiation is directed towards the mask targets and through a portion ofthe mask to the wafer targets.

The change in properties of the reflected light due to scattering ordiffraction may then be used to determine the offset between the patternon the mask and the pattern on the wafer. The wafer is then movedrelative to the mask (or vice versa) to achieve the desired offset. Amore accurate alignment may then be achieved, rather than withconventional alignment techniques such as direct imaging or moirétechniques. The instrument could be a reflectometer, polarizedreflectometer, spectrometer, imaging reflectometer, imaginginterferometer, or other such instrument as described herein or in theabove referenced provisional applications.

Disposition of Scatterometry Overlay Targets

The accuracy of scatterometry overlay systems can be improved by takingmeasurements at multiple targets located across the surface of interest.In one implementation, the scatterometry overlay system may utilize aplurality of scatterometry targets at various locations across thesurface of interest and for each target the system may make onescatterometric measurement of overlay. In another implementation, thescatterometry overlay system may utilize a plurality of scatterometrytarget areas at various locations across the surface of interest. Thescatterometry target areas comprise multiple targets, each of which canbe measured by the scatterometry overlay system. By way of example, thescatterometry targets or scatterometric target areas may be located atthe corners of one or more devices being formed on a wafer. In addition,the scatterometry targets may generally include a grating structure,which is measurable by the scatterometry overlay system.

The number of targets generally depends on the available space on thesurface of interest. In most cases, the targets are placed in the scribeline between devices on a wafer. The scribe line is the place on thewafer where the wafer is separated into dies via sawing or dicing andthus the circuit itself is not patterned there. In cases such as this,the number of targets may be limited, at least in part, by thenarrowness of the scribeline. As should be appreciated, the scribe linestend to be narrow so as to maximize the amount of devices on the wafer.

In accordance with one embodiment of the present invention, the targetsare strategically placed on the surface of interest in order to overcomeany space constraints while increasing the number of targets. In oneimplementation, at least two targets are placed substantiallycollinearly in a first direction. For example, they may be placedcollinearly in the x-direction or the y-direction. This arrangement maybe useful when confronted with narrow spaces as for example scribelines. In another implementation, multiple targets are disposedcollinearly in multiple directions. For example, multiple targets may bedisposed collinearly in both the x direction and the y-direction. Thisarrangement may be useful at the corner of a device as for example atthe intersection of two scribe lines.

Although the examples given are directed at a Cartesian coordinatesystem as defined on the surface of interest, it should be noted thatthe coordinate system may be oriented arbitrarily on the surface orinterest with the x and y axis being rotated or possibly interchanged.Alternatively or in combination with the Cartesian coordinate system,any other coordinate system may be used such as for example, a polarcoordinate system.

FIG. 9A is a top view diagram of a scatterometric target area 900 havingone or more targets 902, in accordance with one embodiment of thepresent invention. The scatterometric targets 902 are generally providedto determine the relative shift between two or more successive layers ofa substrate or between two or more separately generated patterns on asingle layer of a substrate. By way of example, the scatterometrictargets may be used to determine how accurately a first layer alignswith respect to a second layer disposed above or below it or howaccurately a first pattern aligns relative to a preceding or succeedingsecond pattern disposed on the same layer.

As shown in FIG. 9A, the scatterometric target area 900 includes atleast two substantially collinear targets 902. By collinear, it isgenerally meant that the centers of symmetry for each of the targets 902lie on the same axis 904. By way of example, the axis 904 may be alignedwith a conventional coordinate system (Cartesian, polar, etc.) or somevariation thereof. By placing the targets 902 collinearly, thescatterometric target area 900 does not take up as much width W andtherefore may be placed in constrained places as for example in thescribeline of the wafer.

The targets 902 are generally juxtaposed relative to one another alongthe axis 904. In most cases, the juxtaposed targets 902 are spatiallyseparated from one another so that they do not overlap portions of anadjacent target 902. Each of the targets 902 is therefore distinct,i.e., represents a different area on the substrate. This is typicallydone to ensure that each of the targets 902 is properly measured. Thespace 906 between targets 902 produces distortions in the optical signaland therefore it is excluded from the overlay calculation. The size ofthe space 906 is typically balanced with the size of the targets 902 soas to provide as much information as possible for the measurement ofoverlay. That is, it is generally desired to have larger targets 902 andsmaller spaces 906 there between. The space 906 between targets 902 maybe referred to as an exclusion zone.

The targets 902 may be widely varied, and may generally correspond toany of those overlay targets that can be measured via scatterometry. Byway of example, the targets 902 may generally include one or moregrating structures 908 having parallel segmented lines 910. Although nota requirement, the segmented lines 910 for the collinear targets 902 aregenerally positioned in the same direction, which may be parallel ortransverse to the axis 904. In most cases, some of the segmented lines910 are perpendicular to the axis 904 and some are parallel to the axis904 to enable overlay measurements in x and y. Furthermore, the targets902 may have an identical configuration or they may have a differentconfiguration. Configuration may for example include the overall shapeand size of the target 902 or perhaps the line width and spacing of thesegmented lines 910 associated with the grating structure 908 containedwithin the target 902. Preferably the targets used for the overlaymeasurement in a particular direction, for example x direction, aredesigned to have the same configuration except for the programmed ordesigned overlay offsets.

The number of targets may also be widely varied. As should beappreciated, increasing the number of targets, increases the number ofdata collection points and therefore the accuracy of the measurement.The number of targets 902 generally depends on the overall size of thetargets 902 and the space constraints in the direction of the axis 904.In the illustrated embodiment, eight side by side targets 902 arepositioned within the scatterometric target area 900. A scatterometrictarget area may be equivalent to an xy scatterometry overlay targetgroup as discussed above.

Using the above mentioned targets 902, scatterometric overlaymeasurements may be made sequentially, one target at a time, to measureoverlay while eliminating effects due to variations in other sampleparameters, such as film thickness. This can be accomplished viacontinuously scanning of the scatterometric target area (including forexample the targets and the spaces there between) or by stepping to eachof the targets. Alternatively, measurements may take place substantiallysimultaneously using two or more scatterometry signal beams for two,more than two, or all targets to increase throughput. The multiplescatterometry signal beams may come from more than one substantiallyindependent scatterometry optical systems, or they may share much of theoptical system, for example they may share the same light source, thesame beam directing optics, or the same detector system.

Although the method described above includes placing the centers ofsymmetry for each of the targets substantially collinear, it should benoted that the centers of symmetry may be offset from the axis so longas a measurable portion of the targets still falls on the same axis.

Furthermore, although the method described above includes placingtargets of similar orientation along the same axis, it should be notedthat some of the targets may be positioned with a different orientation.For example, a first group of the targets 902 may have segmented linespositioned in the x dimension while a second group of the targets 902may have segmented lines position in the y dimension.

Moreover, although the targets 902 are only shown positioned along asingle axis 904, it should be noted that the targets may be positionedon multiple axis. For example, as shown in FIG. 9B, a first group oftargets 902A may be disposed collinearly along a first axis 904A and asecond group of targets 902B may be disposed collinearly along a secondaxis 904B. This implementation permits independent measurement ofoverlay in at least two directions. The first and second axis aretypically transverse to one another and more particularly perpendicularone another. In the illustrated embodiment, the first axis 904Acorresponds to the X-dimension while the second axis 904B correspondsthe Y-dimension. Furthermore, each group consists of four targets 902.This implementation permits independent measurement of overlay in the Xand Y directions.

Further still, although the targets have been described as havingfeatures (e.g., segmented lines) in substantially one direction, itshould be noted that the targets may include features in more than onedirection. In one implementation, for example, one or more of thecollinearly positioned targets include features that permitscatterometric overlay measurement in first and second directions. Byway of example, the features such as the segmented lines may bepositioned in both the X and Y dimensions. In this case, the need fordisposing targets along more than one axis as shown in FIG. 9B may bereduced or eliminated. That is, if each target has features that permittwo-dimensional scatterometric measurements, overlay may be determinedalong both the X- and Y-axes using a single set of targets disposedsubstantially-collinearly along a single axis. Alternatively, one ormore targets may include one or more sub-targets. If the sub-targetshave features that permit two-dimensional scatterometric measurements,the number of targets desirable for a particular degree of measurementaccuracy may be reduced, and the targets may be disposed along a singleline.

Additionally, targets disposed along one or more axis may be used formeasurement of more then one parameter. For example, a first set oftargets may be used for scatterometric measurement of line width alongthe X-axis and a second set of targets may be used for scatterometricmeasurement of line width along the Y-axis. In an alternativeimplementation, scatterometric measurement of line width may beperformed along the X-axis while spectral measurements are performedalong the Y-axis.

Combined CD and Overlay Marks

Scatterometry measurement targets consume a significant area of thewafer for both metrology of CD and overlay. This wafer area becomes veryvaluable as design rule shrinks. Currently, scatterometry overlay marksmay consume>35×70 μm space for each xy scatterometry overlay targetgroup or mark on the wafer. These are used only for overlay measurementsand therefore the wafer manufacturers consider the loss of wafer spaceas undesirable. Therefore, it is desirable to reduce the total waferarea required for measurement targets or measurement features. Changesto optical system design to enable measurements on smaller targets mayresult in greater complexity of the optical system and potentiallycompromise measurement performance. In scatterometry overlay measurementas describe herein, the target area typically consists of four gratingsfor each axis (X and Y). Each of these gratings is typically larger than15×15 μm with a limited opportunity to shrink it further usingconventional techniques. Each grating is composed of a first layergrating (e.g. STI or shallow trench isolation) and a top layer grating(e.g. gate resist). One of the two layers has a programmed offset, whichis typically smaller than the pitch of the top grating. In many casesthe top layer is photoresist. An overlay measurement is achieved byanalyzing the spectra of a reflected light from each of these gratings.

In scatterometry critical dimension (CD) or scatterometry profilemeasurement, the target area typically consists of a single grating orperiodic structure, which may be positioned along either axis (X or Y).In some cases, the target area may include multiple gratings for eachaxis (X and Y). Each of these gratings is typically about 50×50 μm. Themeasurement is typically performed on a single process layer target withno pattern underneath following completion of a L1 patterning step. Thismeasurement is typically done on a photoresist pattern following aresist development step in a lithographic patterning process orfollowing an etch or CMP process in other modules of the waferfabrication process. A CD or scatterometry profile measurement isachieved by analyzing the spectra of a reflected light from thegrating(s) as described in the above referenced U.S. Pat. No. 6,590,656by Xu, et al.

In accordance with one embodiment of the present invention, thescatterometry CD marks and the scatterometry overlay marks are combinedto enable the fab to save wafer space and to print larger scatterometryoverlay marks with no impact on the wafer scribe-line. The combined markis constructed with a scatterometry CD target (which is one continuousgrating taking area of 4 scatterometry overlay gratings) as the firstlayer and scatterometry overlay target patterns (with correspondingshifts regarding to the first layer) as the top layer. This results inzero or minimal additional scribe line space allocated to scatterometryoverlay.

FIG. 12 is a diagram of a combined mark 1200, in accordance with oneembodiment of the present invention. The combined mark 1200 provides forboth scatterometry CD measurement and scatterometry overlay measurementat different steps in the wafer manufacturing process. The combined mark1200 is formed on at least two layers of the wafer, particularly a firstlayer L1 and a top layer L2. The first layer L1 includes scatterometryCD/profile targets 1202 and the top layer L2 includes scatterometryoverlay targets 1204. Although shown as separate layers in the diagram,it should be noted that the scatterometry overlay targets 1204 are builton (over) the scatterometry CD profile targets 1202. The scatterometryCD/profile targets 1202 form an L1 scatterometry CD mark, which can bemeasured to determine CD after formation or processing of the L1pattern. The scatterometry overlay targets 1204 cooperate with thescatterometry CD/profile targets 1202 to form an L2-L1 scatterometryoverlay mark, which can be measured to determine overlay between thelayers after formation of the L2 pattern (which comes after L1 patternformation). As should be readily apparent, this method may be repeatedto produce a layer 2 L2 scatterometry CD/profile target(s) followed by aLayer 3 L3 pattern to create an L3-L2 scatterometry overlay mark ortarget area.

The configuration of the scatterometry CD/profile targets 1202 andscatterometry overlay targets 1204 may be widely varied. In theillustrated embodiment, the scatterometry CD/profile targets 1202disposed on L1 include a first grating 1206 oriented in a firstdirection and a second grating 1208 oriented in a second direction. Thefirst direction may be orthogonal to the second direction. By way ofexample, the first grating 1206 may include vertical lines while thesecond grating 1208 may include horizontal lines. In addition, thescatterometry overlay targets 1204 disposed on L2 include a first groupof gratings 1210 and a second group of gratings 1212. Both the first andsecond groups of gratings 1210, 1212 include one or more gratings 1214.The number of gratings 1214 may be widely varied. In one implementation,both the first and second groups 1210 and 1212 include four gratings1214. The gratings 1214A in the first group 1210 are oriented in thefirst direction, and the gratings 1214B in the second group 1212 areoriented in the second direction. By way of example, the gratings 1214Ain the first group 1210 may include vertical lines while the gratings1214B in the second group 1212 may include horizontal lines.

In order to produce an L2-L1 overlay mark, the first group of gratings1210 is positioned over the first grating 1206 of the CD/profile targets1202, and the second group of gratings 1212 is positioned over thesecond grating 1208 of the CD/Profile targets 1202. This places gratingswith similarly directed lines together, i.e., vertical lines withvertical lines and horizontal lines with horizontal lines. The firstgroup of gratings 1210 cooperates with the first grating 1206 of theCD/profile targets 1202 and the second group of gratings 1212 cooperateswith the second grating 1208 of the CD/profile targets 1202. Thealignment between layers is determined by the shift produced between thecorresponding lines of these cooperating structures. The vertical lines,for example, may be used to determine X overlay and the horizontallines, for example, may be used to determine Y overlay. In analternative embodiment, L1 or L2 pattern may be periodic structurescomprised of line segments, cylindrical holes or features (contact orvia holes in resist or filled contacts, for example), device-likestructures, and the like.

Although the first and second gratings 1206 and 1208 of the CD mark areshown together, it should be noted that they may be placed apart. Whenimplemented apart, the first group of gratings 1210 and the second groupof gratings 1212 would also be placed apart, i.e., the first group ofgratings 1210 goes with the first grating 1206 and the second group ofgratings 1212 goes with the second grating 1208.

The advantages of combining overlay and CD marks are numerous. Differentembodiments or implementations may have one or more of the followingadvantages. One advantage of combining marks is in the ability to reducethe need for additional wafer space for scatterometry overlay targets.Another advantage is that larger scatterometry overlay targets may beallowed if they do not require as much additional scribe line space.Larger scatterometry overlay targets may make optical design or opticsmanufacturing easier and may provide better scatterometry overlaymetrology performance than on smaller scatterometry overlay targets.

Combination of Scatterometry Overlay and CDSEM

The purpose of this embodiment is to enable measurement of criticaldimensions on a semiconductor wafer with an electronic microscope(CD-SEM) and measurement of overlay using scatterometry on the samemeasurement system or using linked measurement systems sharing at leastpart of a robotic wafer handling system. The established methods ofmeasuring critical dimensions and overlay commonly require schedulingand operating separate measurement systems. One disadvantage of theestablished methods of measuring critical dimensions and overlay onseparate measurement systems is the additional time required to scheduleand run separate operations on separate metrology tools. Anotherdisadvantage is the redundancy of common parts and the costs associatedtherewith.

In order to overcome these disadvantages, a metrology system thatcombines Scatterometry Overlay and CDSEM may be provided. In oneembodiment, a scatterometry overlay measurement (SCOL) system isintegrated with a CDSEM system such that the CDSEM and SCOL systemsshare at least part of the robotic wafer handling system and/or datasystems. Alternatively, the CDSEM and the scatterometry overlay systemsmay be separate systems capable of independent operation, but linked insuch a way that they share at least part of a robotic wafer handlingsystem.

In operation, a wafer, a group of wafers, or batch of multiple wafersmay be introduced to the combined metrology system by loading the wafercontainer onto the robotic wafer handling system dedicated to thiscombined metrology system. Measurement recipes may be selectedspecifying CDSEM measurements on some or all of the wafers andscatterometry overlay measurements on some or all of the wafers. TheCDSEM measurements and the SCOL measurements may be specified togetherin one or more recipes, or may be specified in separate recipes. TheCDSEM and SCOL measurements may be done on the same wafers or ondifferent wafers or on some of the same wafer and some different wafers.The CDSEM and SCOL systems may operate in parallel, or in series.

One example of the combined metrology system would be integration of ascatterometry system capable of scatterometry overlay measurements (suchas a spectroscopic ellipsometer, spectroscopic polarized reflectometer,or +/−1 order diffraction scatterometer) inside a CD-SEM such as any ofthose manufactured by KLA-Tencor of San Jose, Calif. Another example ofa combined metrology system would be a linked system comprising ascatterometry overlay system, a CDSEM such as any of those manufacturedby KLA-Tencor of San Jose, Calif., a robotic handler, and a waferscheduling system. Communication to factory automation and/or factoryinformation, and/or factory process control systems may be throughseparate communication or automation systems or may be at leastpartially or completed shared.

One advantage of the combined CDSEM and SCOL metrology system is thereduction in overall time required to complete scheduling and/orperforming the CDSEM and scatterometry overlay measurements. At leastone queue delay time may be eliminated. Performing CDSEM and overlaymeasurements in parallel can save at least part of the time required forseparate measurement operations.

FIGS. 13A-13D show variations of a combined metrology tool 1300, inaccordance with several embodiment of the present invention. In all thefigures, the combined metrology tool 1300 includes a robotic waferhandling system 1302, a critical dimensioning scanning electronmicroscope (CD-SEM) 1304, a scatterometry overlay (SCOL) measurementinstrument 1306, a wafer load position A 1308 and a wafer load positionB and 1310, respectively. The robotic wafer handling system 1302 isconfigured to transfer wafers to and from the CD-SEM 1304 and SCOLmeasurement instrument 1306 as well as to and from the wafer loadpositions A and B 1308 and 1310. The critical dimensioning scanningmicroscope 1304 is configured to measure the critical dimensions thatmay include, for example, linewidth, top linewidth, via diameter,sidewall angle and profile. The scatterometry overlay measurementinstrument 1306 is configured to measure the overlay as for examplebetween two layers disposed on the wafer. The wafer load position A andwafer load position B are configured to hold one or more wafers. In mostcases, they hold a plurality of wafers. The wafers may be from the samelot or from a different lot.

In FIGS. 13A and D, the CD-SEM 1304 and the SCOL measurement instrument1306 are separate systems that are integrated via the robotic waferhandling system 1302. In FIG. 13B, the SCOL measurement instrument 1306is integrated into the CDSEM 1304. In FIG. 13C, the SCOL measurementinstrument 1306 is integrated into the robotic wafer handling system1302.

In one operation, some of the wafers from wafer load position A and/or Bhave critical dimensions measured at the CD-SEM and thereafter haveoverlay measured at the scatterometry overlay measurement instrument.The wafer can be measured by both processes without being removed fromthe system, i.e., the wafer handling as well as the throughput issuesassociated therewith are reduced. In another operation, some wafers fromwafer load position A and/or B have critical dimensions measured at theCDSEM and some other wafers from wafer load position A and/or B haveoverlay measured on SCOL measurement instrument. In any of theseoperations, the CDSEM and SCOL measurement instrument can proceedindependently and simultaneously.

FIG. 14 is a flow diagram 1400 using a combined metrology tool, inaccordance with one embodiment of the present invention. The methodgenerally includes step 1402 where a group of wafers are received by themetrology tool. By way of example, the wafers may be a wafer lot that isloaded at position A in FIG. 13. Following step 1402, the process flow1400 proceeds to step 1404 where the critical dimensions of a wafer fromthe group of wafers is measured. By way of example, the criticaldimension measurements may be performed by a CDSEM as for example theCDSEM shown in FIG. 13. The process flow 1400 also proceeds to step 1406where the overlay of a wafer from the group of wafers is performed by aSCOL measurement instrument as for example the instrument shown in FIG.13. Steps 1404 and 1406 may be performed at the same time on differentwafers. Steps 1404 and 1406 may be performed on the same wafer in asequence of operations, as for example, from CD to overlay or fromoverlay to CD. The transferring of the wafer may for example beperformed by the robotic system shown in FIG. 13. When all themeasurements are performed, the process flow proceeds to step 1408 wherethe group of wafers are released from the metrology tool.

Uses of Scatterometry Overlay Data

The overlay results obtained with scatterometry overlay techniquesdescribed herein, including the linear differential method andphase-detection algorithms, may be used to calculate corrections to thestepper settings to minimize overlay error. These calculated correctionsfor lithography steppers or scanners are commonly referred to as“stepper correctables.” The stepper correctables obtained fromscatterometry overlay measurements may be used as inputs to the stepperto minimize overlay error for subsequent wafer processing. The overlayerrors or stepper correctables obtained from scatterometry overlay maybe input to an automated process control system which may then calculatea set of stepper corrections to input to the stepper to minimize theoverlay errors for subsequent wafer processing. The overlay errors,stepper correctables, or calculated worst overlay errors on the waferobtained with scatterometry overlay may be used to disposition productwafers to decide if the wafer requires rework or meets overlayrequirements for further wafer processing.

Combination of Scatterometry Overlay and Other Metrology or InspectionMethods

Scatterometry overlay may be combined with scatterometry profile orscatterometry critical dimension systems, or other semiconductormetrology or inspections systems. Scatterometry overlay may beintegrated with a semiconductor process tool, for example a lithographyresist process tool (also known as a resist track). Integration ofmetrology systems with process systems and combinations of metrologysystems are described in (1) U.S. patent application, having U.S. Pat.No. 09/849,622, filed 04 May 2001, entitled “METHOD AND SYSTEMS FORLITHOGRAPHY PROCESS CONTROL”, by Lakkapragada, Suresh, et al. and (2)U.S. patent, having U.S. Pat. No. 6,633,831, issued 14 Oct. 2003,entitled “METHODS AND SYSTEMS FOR DETERMINING CRITICAL DIMENSION AND ATHIN FILM CHARACTERISTIC OF A SPECIMAN”, by Nikoonahad et al, whichapplications are incorporated herein by reference in their entirety.

Scatterometric Overlay with Crossed Gratings

Scatterometry overlay line targets with L1 and L2 line elementsperpendicular to underlying line grating L0 (or any number of underlyinggratings). In this case the scatterometry overlay signal is notsensitive to the positions of L1 and L2 with respect to L0. Oneadvantage is to reuse wafer area already used for scatterometry profiletargets in a previous process layer. For example, targets fordetermining overlay in two different sets of layers can be stacked atopone another.

In another embodiment, scatterometry overlay line targets with L1 and L2line elements perpendicular to underlying line grating L0 where L0 isone or more material(s) (copper damascene structure, for example), wherethe L0 pitch and line width are such that the scattered signal issubstantially less sensitive (over at least part of the spectrum orsignal conditions) to structures (e.g. film thickness or otherstructures) beneath L0 than would be the case in the absence of L0, areused. One example of an L0 structure that has these properties is acopper damascene line grating with 200 nm pitch, 100 nm line width, and500 nm height. Proper choice of pitch, line width, etc. can produce a L0structure that screens the underlying structures and produces a signalthat is less sensitive to underlying features for at least some signalwavelengths, polarizations, etc. Optical simulations may be used todetermine the preferred L0 properties including pitch and line width.

FIG. 15 is a perspective diagrammatic view of overlay line targets withL1 and L2 line elements perpendicular to underlying line grating L0 inaccordance with one embodiment of the present invention. As shown,overlay target structures are formed within layer 2 (L2) 1502 and theseL2 structures 1502 are positioned over overlay target structures whichare formed in layer 1 (L1) 1504. Film 1506 is disposed between L1structures 1502 and L2 structures 1504. L1 structures are also formedover underlying structures in layer 0 (L0) 1508 with film 1510 disposedbetween the L1 and L0 structures.

In one implementation, the L0 structures 1508 are perpendicular to theL1 structures 1510 so as to not substantially affect the measurement ofstructures L1 or L2. That is, when incident radiation 1512 impinges onL1 and L2 structures 1504 and 1502, scattered radiation 1514 is notaffected significantly by positioning of the L2/L1 overlay targetstructure above L0. Additionally, L0 structures 1508 may be formed fromone or more material(s) that form a barrier or screen over theunderlying layers and structures. For instance, underlying structures1516 (and film 1518) do not significantly affect the scattered radiation1514 for at least part of the signal spectrum or at least one of theoptical signals. Film 1518 is also typically disposed between L0structures 1508 and underlying structures 1516.

Although the foregoing invention has been described in some detail forpurposes of clarity of understanding, it will be apparent that certainchanges and modifications may be practiced within the scope of theappended claims. For example, although the terms wafer or sample wereused throughout (e.g., semiconductor), it should be noted that any typeof workpieces may be utilized, such as substrates, photomasks, flatpanel displays, electro-optic devices, and the like which are associatedwith other types of manufacturing. The term “stepper” was usedthroughout as an example to generically represent lithography systems inuse or in development in the semiconductor industry and relatedindustries and is not a limitation on the lithography systems which mayinclude steppers, scanner, imprint lithographic systems, electron basedlithographic patterning systems, ELV based lithographic patterningsystems and the like. Therefore, the described embodiments should betaken as illustrative and not restrictive, and the invention should notbe limited to the details given herein but should be defined by thefollowing claims and their full scope of equivalents.

1. A method of determining an overlay error between two layers of amultiple layer sample, the method comprising: for each of a plurality ofperiodic targets that each have a first structure formed from a firstlayer and a second structure formed from a second layer of the sample,measuring a plurality of optical signals at a plurality of incidentangles, wherein there are predefined offsets between the first andsecond structures; and determining and storing, in memory, an overlayerror between the first and second structures by analyzing the measuredoptical signals at the plurality of incident angles from the periodictargets using a scatterometry overlay technique based on the predefinedoffsets without using a calibration operation or a model-basedregression technique, wherein the scatterometry overlay technique is aphase based technique that includes representing each of the first andsecond optical signals as a set of periodic functions having a pluralityof known parameters, and an unknown overlay error parameter andanalyzing the set of periodic functions to solve for the unknown overlayerror parameter to thereby determine the overlay error.
 2. A method asrecited in claim 1, wherein the overlay error is determined withoutcomparing the measured optical signals to calibration data.
 3. A methodas recited in claim 1, wherein the plurality of measured optical signalsfor each target are obtained simultaneously.
 4. A method as recited inclaim 3, wherein the plurality of measured optical signals for eachtarget are obtained simultaneously by a beam profile reflectometer.
 5. Amethod as recited in claim 3, wherein the plurality of measured opticalsignals for each target are obtained simultaneously by an OpticalFourier Transform reflectometer.
 6. A method as recited in claim 3,wherein the plurality of measured optical signals for each target areobtained simultaneously by an Optical Fourier Transform ellipsometer. 7.A method as recited in claim 3, wherein the plurality of measuredoptical signals for each target are obtained simultaneously by anOptical Fourier Transform polarized reflectometer.
 8. A method asrecited in claim 3, wherein the plurality of measured optical signalsfor each target are obtained simultaneously by a simultaneous, multipleangle of incidence ellipsometer.
 9. A method as recited in claim 8,wherein the ellipsometer includes a plurality of detector elements thatare each arranged to detect a one of the plurality of measured opticalsignals for each target.
 10. A method of determining an overlay errorbetween two layers of a multiple layer sample, the method comprising:for each of a plurality of periodic targets target that each have afirst structure formed from a first layer and a second structure formedfrom a second layer of the sample, measuring a plurality of opticalsignals at a plurality of incident angles, wherein there are predefinedoffsets between the first and second structures; determining andstoring, in memory, an overlay error between the first and secondstructures by analyzing the measured optical signals at the plurality ofincident angles from the periodic targets using a scatterometry overlaytechnique based on the predefined offsets without using a calibrationoperation or a model-based regression technique, wherein each firststructure has a first center of symmetry and each second structure has asecond center of symmetry and wherein the first center of symmetry andthe second center of symmetry for each target are offset with respect toeach other by a selected one of the predefined offsets; and comparingthe measured optical signals to theoretical data, using a model-basedregression technique, to thereby determine and store, in memory, asecond overlay error between the first and second structures.
 11. Amethod as recited in claim 10, further comprising comparing the firstoverlay error to the second overlay error to thereby adjust the modelused to generate the theoretical data.